Amorphous hydrogenated silicon-tin alloys have been prepared by if sputtering using a target of silicon with stripes of tin placed over the silicon surface in order to attain the appropriate silicon-tin composition. It is shown that for the composition range investigated, 0 ≤ 5 x ≤ 0.51, the addition of Sn moves the conduction band edge, thereby closing the optical band gap. The dependence of Eg on x can not be described by a single linear relationship. Sn also creates dangling bonds which further increase the material disorder. The electrical dc dark conductivity increases less than expected with increasing Sn content based on a simple decrease in band gap. This and the temperature dependence of the dark conductivity suggest a transition from extented state free-carrier conduction to localized state hopping conduction. The bonding between Si and H competes with that of Sn and H. Photoluminescence measurements show the presence of defect state radiative recombination. Calculations of optical band gaps, using the Coherent Potential Approximation with diagonal disorder and the off-diagonal disorder being treated within the virtual crystal approximation, are in excellent agreement with the experimental results.