Honoraria are payments made for services for which fees are not traditionally required, payment for services not related to an individual's official duties, or payments granted in recognition of a special service such as a speech.  When the individual is a US citizen it is generally permissible to pay an honorarium.  For foreign nationals it is more complicated and takes far longer to iron out the details than when paying a US citizen.  When paying a foreign national an honorarium, it is extremely important to start the process early and think about all of these issues well before the foreign national visitor is invited.  See Payments to Non-Resident Aliens (NRAs) and Foreign Entities "Who are Legal to Pay"

Please see "A Guide to Paying Honoraria at Princeton University" and the section of this website titled Contracts for details and contacts of those who are involved in the process and can assist you.

In lieu of paying honorarium it might be advisable to reimburse individuals for expenses or pay for their expenses directly.  This option is always tax free whether they are US citizens or not.