Abstract: A gaseous contaminant dosimeter apparatus for collecting, for subsequent analysis, a gaseous contaminant in proportion to its average concentration in the ambient atmosphere over a predetermined collection period. The gaseous contaminant dosimeter apparatus includes a closed chamber containing a medium capable of chemically or physically combining with the selected gaseous contaminant, and porous diffusive material formed from a material through which the contaminant may diffuse at a rate that is proportional to its atmospheric concentration and substantially unaffected by convective movements of the ambient atmosphere, is mounted in fluid impeding relation with the chamber. The diffusive material includes at least two layers of microporous membrane material having a multiplicity of pores which individually have effective cross-sectional dimensions that are smaller than the mean free path length of the contaminant, the membrane material being mounted on the opposite sides of a porous support substrate.