I found this out the hard way today when I called Continental to see about using my miles to upgrade for one leg of my flight. The representative told me that essentially, I can't use my miles to get upgraded on a United operated flight if I bought my ticket on Continental website, and vice-versa (i.e. if I bought my ticket on United and it's a Continental operated flight). I asked her where was it explained/written in the website, and she said that it isn't, but it's just something the system won't let them do and so the representatives on both airline are aware of this, but I don't think passengers are aware of this (at least I wasn't). She did say come march when the integration is done, then this restriction "should" go away. But in the meantime, this really doesn't do me any good. Here is more detail: I had bought a round trip last week on Continental for Newark to Vienna. For the departure flight, I am actually flying from EWR -> Zurich on a United operated flight and then at Zurich->Vienna it's an Austrian airline flight. On the return, it's Zurich -> LHR (on Austrian air) and then LHR->EWR on Continental operated. I am a Onepass Platinum elite as well as United 1K and had the option to buy this either on Continental or United website but I generally prefer Continental website's reservation system more, especially in terms of how they display elite miles you can earn (with partner airlines, they tell you how much actual elite miles you'll earn depending on the fare class). So I was surprised today when I called Continental to see about using miles to get my flight from EWR->Zurich upgraded to biz class and the representative took one look at the reservation and said, sorry, no can do. And went on to say it's because of systems limitations which still don't treat united and continental as one company, despite all the hoopla about them being one company for the last 1+ year. So lesson learned on this the hard way. I guess if I plan on flying again on either untied/continental and it involve a flight operated by the other carrier, I should choose to get it ticketed at the airline where I think I would like to be able to use miles to upgraded. But flyers beware!