The misfit dislocation structure of vicinal InP-on-GaAs heterostructures is studied by transmission electron microscopy (TEM). An island type growth is identified. The misfit stress is not fully relaxed at the interface. X-ray measurements on strain relaxation and epilayer misorientation are also reported, and the latter results are explained with the asymmetric introduction of 6Ø° dislocations at island edges. Comparing the results, it is concluded that x-ray data supply additional, although indirect, information on initial growth which is hardly detectable by TEM.