There is a possibility that wearing traditional sampling pumps could affect worker behaviour and undermine the validity of occupational hygiene exposure measurement. Sampling pumps are bulky and can be noisy, and if these characteristics alter work patterns sufficiently exposures may be modified. This paper describes a carefully controlled pilot study designed to test the hypothesis that the wearing of personal sampling pumps may affect workers' exposure. Comparisons were made using pump-based systems and diffusive samplers to measure organic vapours. Diffusive samplers were chosen as controls because they are much less bulky and therefore less likely to affect behaviour. A carefully constructed sampling strategy, involving measurements on between 13 and 20 workers on 6 working days, was used at five sites. On alternate days both sampling methods were used, side-by-side, on the workers and on the remaining days only diffusive samplers were worn. The sites chosen had minimal day-to-day variation in environmental conditions. The results showed that at one site the concentrations were approximately 50% higher on the days when pumps were worn compared with other days. At the remaining sites no significant differences were found. From the characteristics of the work at the site where differences were observed it is suggested that pumps may affect measured concentrations when people have flexible work patterns and spend part of their time seated and/or the work is in quiet areas.