A method of preparing thin, uniform 35S-sources for use in β-spectroscopy is described. A short review of the techniques previously used is given, and their main properties are discussed. The new method presented here consists of exposing a thin layer of vacuum-evaporated copper to 35S-labeled H 2S gas. A yield close to 100% can be achieved. The properties of samples made with non-active H 2S gas were investigated with electron microscopy and X-ray diffraction. A 35S-source with an activity of 5.3 mCi was prepared on a 0.2 μm thick Kapton foil. From measurements of the effective half-life, the source was found to be chemically stable in vacuum.