Some time ago trifoliate oranges, kumquats, eremocitrus and microcitrus species weren't considered as true citrus. So hybrids between them and a "true" citrus (like lemon, orange etc.) were called intergeneric. Now all of them are believed to be true Citrus species and names such like Poncirus trifoliata or Eremocitrus glauca are no longer correct.

I've found one document about crossing Citropsis schweinfurthii and ‘Nanpu’ tangor / ‘Chandler’ pummelo. It states that some seedlings resulted from these crosses, but I can't find more info about them on the Internet.

So I wonder, if there exists any viable intergeneric citrus hybrid? Is it potentially possible to cross a citrus and, for example, murraya? Or Hesperthusa species? I don't think it would be easy, but I'm curious: has anybody ever done this?

I don't believe it is possible.

Try searching for Citrus INTRAGENERIC hybrids, and also Citrus Intrageneric somatic hybrids ( where non reproductive cells can be fused to form the hybrid, instead of crossing the pollen and ovum to form seed in fruit. )
Looks like Citrus X Murraya has been tried by cross pollination. and Citrus X Clausena has been done as somatic hybrid.
Somatic hybrids dont require pollen and ovum to be compatible, as would be required with trying to cross pollinate Citrus and its relatives.
This method is now more likely to be attempted to get wide crosses with Citrus and another related Genus.

Thanks for the answers. Yeah, the modern breeding methods allow scientists to get many more hybrids than it would be possible with cross-pollination only. It's nice that Citrus x Murraya has been tried with cross-pollination, pagnr. I'll try to make such a cross when my murraya flowers.

Intergeneric hybrids might also be possible with classical technics. Once again I remember what Ivan Mitschurin did in Russia in order to cross different species and genus (for example Sorbus and Aronia). His procedure was the following:
Start with seedlings. Any other plant doesn't work. In or after (I don't remember) one year from germination cross graft the seedlings. Let them grow. Then graft the twigs of the seedlings into an aduld tree of the opposite species of each. Wait for the first year of blooming and cross. In the second year it is already impossible. This was Mitschurins experience and techics. He emphazised that such a cross is only possible if you start with seedlings due to the greater flexibility of seedlings as compared to mature plants. And he emphasized that crosses will only work with the very first flowers of a plant. Later, he claimed, the plants are not flexible enough.
If a cross does not work even with these trixs he said you should give a little bit of the pistil secretum of the pollen parent on the pistil of the mother plant. And if that does not help also then graft the pistil of the father plant in place of the mother plant's pistil.
By the way, Mitschurin made another interessing observation: Some of the hybrids turned out to be sterile for many years but suddenly developed a lot of viable seeds in one year and from that forever. He took that as another proof that plants learn by experience.
Mitschurin's theories might be contentious. But one thing is very clear: He was extremly successfull.

Mitschurin mainly made his experiments with members of Roseaceae (the classical European fruit trees). It still has to be proven that it also works with Citrus plants and its relatives. But I would wonder if not.

Thanks Till, your post reminded me of something I read about techniques for overcoming cross pollination incompatability in breeding flowers and vegetables.
When trying to cross species from the same genus, or closely related genus, pollen incompatability could be overcome by either using immature pollen, pollinating before the fem flower naturally opened, ie before pollen incompatability fully developed. I think other similar tricks were also mentioned.
Sounds like your immature seedling flowers etc , and older hybrid flowers could have more tolerant pollination incompatability status than the same plant at other maturity stages ?
I will try to find the article again.

This is very encouraging. Do you think it might be possible to cross a citrus with the Northsky blueberry which is hardy to -43C/-45F? Or are the two species just too far apart to be able to attempt that?

Thank you, pagnr, for your information, too. That was new to me.

I found a mistake in my post: INSTEAD OF "STEMEN" I SHOULD HAVE WRITTEN "PISTIL". I am sorry for my English. I used a word that does not exist. But I have corrected it now by re-editing my post.

Couldn't it be interesting to cross Poncirus with some distantly related members of the subtribe Citreae (see the list below in http://de.wikipedia.org/wiki/Rautengew%C3%A4chse)? Maybe that helps crossing out the bad taste of Poncirus. But well, this goal has already been archieved in crosses with Citrus...[/quote]

First fruiting intergeneric hybrids between Citrus and Citropsis obtained by "natural ways".

Any of you subscribes to the "Journal of the American Society for Horticultural Science"? If yes, PM me. Thanks.