Lattice matched InP based GaInAs(P) heterostructures grown by AP-MOVPE have been examined by TEM. Although interfaces appear compositionally abrupt to 1 nm or better, undulations are sometimes observed at GaInAs(P) to InP interfaces that appear to be associated with gas switching procedure. Where this phenomenon is observed we have investigated the influence of growth pauses and growth rate. We find that suitable choices of both these parameters can be used to advantage in improving interface flatness.