Ap-type pseudo-binary alloy semiconductor, Pb0·3Sn0·7Te, has been prepared fromp-type specimens of PbTe and SnTe and lattice constants determined with an accuracy of 0.0001 nm. Vacuum annealing of Pb0·3Sn0·7 Te reveals two new x-ray powder diffraction lines bearing indices (444) and (800), while others become more sharp, CuKa-doublets get clearly resolved and the lattice constant is increased by ∼ 0·0002 nm. Slight deviation from Vegard’s law linearity is observed showing that the sample must be considered as ternary in nature. Thin films deposited on mica and glass substrates kept at room temperature are found to have a little higher SnTe content. The effective carrier concentration calculated from Hall measurements at room temperature is ∼ 3·4×1026 m−3.