A passive sampling unit based on molecular diffusion of the gas to be measured has been designed for use as a personal monitoring device. To demonstrate the applicability of the approach, the sampler was initially used to measure water vapor concentration in the air; the results showed good accuracy and reproducibility. It was then tested with sulfur dioxide as a means of measuring time-weighted average exposure to this gas. The quantity of gas transferred by diffusion from the environment through an orifice of known dimensions into a chamber maintained at substantially zero concentration by a suitable collecting medium can be used as the basis for calculating average concentration during the time the sampler is in the environment. Calculated and observed values for chamber sulfur dioxide concentrations from 10 ppm to about 0.1 ppm agreed very well over a considerable range of orifice dimensions.