It is often assumed1-4 that the pH of natural rainwater is controlled by the dissociation of dissolved CO2, has a value of 5.6, and that decreases below this are due to the addition of acidic components by human activity. However, decreases could be due to the removal by rainwater of naturally occurring acids from the air (notably H2SO4 in the natural portion of the sulphur cycle). Consideration of the cycling of water and sulphate through the atmosphere and the amount and composition of sulphate aerosol expected to be scavenged by a given amount of cloud water in remote locations indicates that, in the absence of basic materials (such as NH3 and CaCO3), average pH values of ~5 are expected to occur in pristine locations. This value must vary considerably due to variability in scavenging efficiencies as well as geographical patchiness of the sulphur, nitrogen and water cycles. Thus, pH values might range from 4.5 to 5.6 due to variability of the sulphur cycle alone. Because of widespread concern regarding the acidification of rain, it is important to understand the factors controlling the pH and composition of natural rainwater. We suggest here that there are integral constraints imposed by (1) the requirement for mass continuity in each elemental cycle and (2) the relative concentrations of soluble species and liquid water in cloudy air; these factors must be mutually consistent.