Tin selenide thin films have been grown by molecular beam epitaxy on GaAs (111)B substrates at a growth temperature of 150 degrees C, and a microstructural study has been carried out, primarily using the technique of transmission electron microscopy. The Se:Sn flux ratio during growth was systematically varied and found to have a strong impact on the resultant crystal structure and quality. Low flux ratios (Se:Sn=3:1) led to defective films consisting primarily of SnSe, whereas high flux ratios (Se:Sn >= 10:1) gave higher quality, single-phase SnSe2. The structure of the monoselenide films was found to be consistent with the Space Group Pnma with the epitaxial growth relationship of [011](SnSe)//1 (1) over bar0](GaAs), while the diselenide films were consistent with the Space Group P (3) over bar m1, and had the epitaxial growth relationship [2 (1) over bar(1) over bar0](SnSe2)//[1 (1) over bar0](GaAs). (C) 2016 Published by Elsevier B.V.