GaInAsP and GaInAs epitaxial layers grown on Fe-doped InP substrates of {100} orientation have been studied using cross-sectional and plan-view transmission electron microscopy. Studies of the epilayer surface regions of LPE Ga[0.03]In[0.97] As[0.07]P[0.93] heavily doped with Mn or Mn and Ge have revealed at least seven precipitate types when classification is made according to precipitate morphology, configurations and associated strain fields. The origin of some of these second phase particles is shown to be iron-phosphide precipitation resulting from iron diffusion from the substrate. Furthermore, in a LPE Ga[0.47]In[0.53]As layer co-doped with Mn and Ge four distinct precipitate types were observed. Speckle and long wavelength microstructures in GalnAs grown by LPE, MOCVD and MBE have been studied. A long wavelength microstructure lying along directions which is observed in some MBE GalnAs layers is associated with reduced electron mobilities. In MOCVD GalnAs a long wavelength microstructure lying along directions was seen to vary as a function of distance from the interface. The presence and wavelength of the speckle contrast is shown to be independent of the growth technique employed. Some MBE GalnAs layers also reveal an island contrast along orthogonal directions. Undoped MOCVD GalnAs epilayers with large compositional fluctuations approximately half way through the thickness of the epilayer or small compositional fluctuations near the epilayer surface have been studied. These perturbations in composition are associated with a dislocation cell structure and a cross-hatch of dislocations along directions respectively. A variety of defects have been identified and the microstructure correlated with Sputter Auger profiles and depth-resolved Hall profiles. Cliff-Lorimer k-factors have been determined experimentally and theoretically for Ga, As and P relative to In and extinction distances for GalnAs and InP calculated.