Thep, T, xdiagram of the Sn‐S system was determined especially in the region of the compound SnS. The pressure of S2in equilibrium with SnS and a liquid phase was found to extend over several decades up to 25‐mm Hg at the ``Sn‐rich'' side, whereas at the ``S‐rich'' side the S2pressures in equilibrium with solid SnS and a liquid phase lie between 25‐mm Hg and 100‐mm Hg. It was shown that the existence region of solid SnS very probably lies entirely at the excess sulfur side. The hole mobility in a plane perpendicular to thecaxis, ≈90 cm2/v sec at room temperature, was proportional toT−2,2for higher temperatures. The mobility in the direction of thecaxis was about five times smaller. Reversible annealing effects were found for temperatures above 200°C which could be explained by assuming association of neutral Sn vacancies. Absorption measurements showed that the edge absorption is due to indirect transitions. The bandgap was 1.08 ev at 300°K and 1.115 ev at 77°K. Interband transitions in the valence band were also found. The effective charge of the atoms (e*=0.7e0) and the effective masses of the holes in the three principal crystal directions (ma*=mb*=0.20m0;mc*≈m0) were determined from reflection measurements in the infrared. From these values and the value for the density of states mass obtained by means of the Seebeck effect (md*≥0.95m0), the number of equivalent maxima of the valence band was found to be at least four.