A TEM investigation of MOVPE grown Zn3As2 revealed the presence of a thin epitaxial Zn3P2 intermediate layer between the InP substrate and the Zn3As2 overgrowth. A model of the orientation relationships between Zn3As2, Zn3P2 and InP is presented. The origin of the Zn3P2 layer was ascribed to the diffusion of Zn atoms into the InP substrate, aided simultaneously by P diffusion from the substrate. A simulation of the Zn3As2 lattice image is presented for the first time, together with a high resolution transmission electron microscopy (HRTEM) image of the Zn3As2/Zn3P2 interface.