Strain relaxation during the growth of InAs thin films on GaAs substrates by molecular beam epitaxy has been studied by scanning tunneling microscopy (STM). A two-dimensional growth mode operates for InAs layers grown on both GaAs(110) and GaAs(111)A. Detailed STM profiles of the InAs surfaces reveal small relaxation phenomena that are related to the presence of misfit dislocations at the buried heterointerface. The origin of these surface features is discussed, and their utility in studying the relaxation of strained semiconductor thin films is demonstrated.