The atmospheric relative humidity is one of the factors that is generally stated as affecting the activity of a vapor phase disinfectant. Data are presented to show that, in beta-propiolactone BPL vapor disinfection, the important factor is, in reality, the moisture content and location of water in the cell and not necessarily the atmospheric RH. Previous studies revealed that only about 50 of the bacterial spores equilibrated to 45 RH were killed when exposed at the same RH to BPL vapor. On the other hand, all the spores equilibrated to and then exposed at 75 RH to BPL were readily killed. Studies presented in this paper show that spores equilibrated to 98 RH are killed by BPL at 45 RH, but only 99 of the spores equilibrated to 75 RH are killed by BPL at 45 RH. Data also show that, to be killed, desiccated spores must be exposed to BPL at higher humidities than would be required if the spores had not been desiccated.