The growth by molecular beam epitaxy of thick zinc blende (ZB) MgS on ZnSe buffer layers has been investigated by atomic force microscopy, X-Ray Diffraction and RHEED for samples grown at 240 °C. Two different populations of pits are observed on these samples, neither of which have been observed previously on samples grown at 270 °C. Type A pits are found on samples grown using the largest Mg fluxes. They are as deep as the MgS layer and appear to nucleate at the underlying surface. Reducing the Mg flux eliminates the type A pits and produces shallow type B pits which are possibly caused by rocksalt phase inclusions. Samples grown with the low Mg flux do not have an increased residual zinc incorporation, which is in agreement with a previously suggested model of the MgS growth mechanism. Single MgS ZB layers up to 64 nm thick can be produced using a low Mg flux before conversion to the rocksalt phase is observed. This thickness is increased using a ZnSe/MgS buffer layer. Optimizing the buffer layer thickness, allows 200 nm thick layers of ZB MgS to be produced.