Samples of CdSxTe1-x were used as a model substance in order to investigate crystal growth from the vapour, twinning phenomena and the electrical properties of grain boundaries. A self-selecting vapour growth method produced solid solutions of CdSxTe1-x (x ≈ 0.067) with a compositional uniformity of less than 0.002x (that is, less than 0.0013) as determined by X-ray diffraction and photoluminescence spectroscopy. The crystals were highly twinned, and statistical tests showed that the twin boundaries were spatially correlated. A model for twin formation was proposed which was based upon nucleation and grain size development. Band-bending at grain and twin boundaries in CdTe and in CdSxTe1-x was measured by using the scanning electron microscope remote electron beam induced current technique. Boundaries in CdTe had downward band-bending, but those in CdSxTe1-x had upward band-bending. The decoration of grain boundaries with Te inclusions reduced the band-bending effect in CdTe. It was postulated that grain boundary passivation in CdTe/CdS solar cells was caused partially by enrichment of the grain boundaries in CdTe with CdSxTe1-x, and partly by the Te enrichment which occurred during processing.