Operations constraints can be expected in any mission and are taken into account during mission preparation. However, practical experience needs to be gained to fully assess those constraints in order to align the operation concept accordingly. This paper addresses the operations constraints experienced by the payload operations team while continuously operating external ESA payloads (SOLAR by B.USOC, and EuTEF by Erasmus USOC) on ISS/Columbus for a period of over 1.5 years. Constraints covered by the paper include issues related to the ISS attitude and orbit, availability of the various ISS resources, and various types of events that potentially interfere with payload operations (ISS thruster firings, vehicle (un)dockings, crew EVAs, ISS maintenance and anomalies, and other ISS operations). This is complemented by an overview of the operations products needed for dealing with these constraints in operations, and some practical issues that were experienced. This overview could be helpful to teams planning operations of external payloads on ISS, in order to properly prepare for the kind of constraints and situations they will typically encounter while operating their payload. Several of the constraints also apply to internal payloads, though additional constraints are likely to apply there. The SOLAR operations are performed from the B.USOC by the Belgian Institute for Space Aeronomy (IASB/BIRA) with Belgian company Space Applications Services, whilst EuTEF was operated from the Erasmus USOC located at ESA/ESTEC by Nederlandse Luchten Ruimtevaartlaboratorium (NLR) jointly with Space Applications Services, both Centers being under contract of the European Space Agency. © 2010 by Space Applications Services.