A diffusion sample collector for gaseous media, consisting of a transparent container open at both ends, which contains a collecting layer and has at its two open ends permeable retaining elements. The collector is improved in such a manner that the presence of hazardous substances in the surrounding air to be analyzed can be recognized during the collecting of the samples in order that an unnecessary analysis of the collecting layer can be prevented in the absence of hazardous substances and also gas eruptions can be noticed immediately. For this latter purpose, an additional layer is provided in the container, which is separated from the collecting layer by a dividing wall and is in contact with the gas to be analyzed through the opposite end and its retaining element.