To help others who might read this thread in the future: At many US airports, international flights operated by US-based airlines DO depart from what are traditionally considered to be "domestic terminals." In Chicago, for example, I believe that all of United and American's international flights depart from T1, T2 and T3. So if you're able to land on your inbound flight and remain airside when moving between gates, you could theoretically check in at the international departing gate. However...in this day and age there's virtually no reason to check in at a gate -- it should be done online as far in advance as possible, or at the check-in counter at your initial departure city. (Keep in mind there are cut offs for checking in, so you run the risk of cutting things close if you're running late.) If you do in fact get to the gate without having checked in, you may find that gate agents are tied up processing earlier departures. However, many airlines have also airside customer service desks and check-in kiosks you can use.