Cadmium sulfide particles were prepared by precipitation from acid solution. A radiotracer technique with109Cd was applied to measure the solubility of cadmium sulfide at various pH's. Filtration, centrifugation, ultracentrifugation, and dialysis were used to separate the particles from the solution. Only the last two techniques proved to be successful. The solubility of cadmium sulfide in water (pH=7) is found to be: 7.9·10–5 mol·l–1 in contrast with the literature value of 9.0·10–6 mol·l–1. At low pH (1–4), the solubility agrees fairly well with the solubility calculated on the basis of generally accepted values for the solubility product and for the various complex formation constants, while at high pH (4–14) the solubility is higher than expected.