A microprocessor-based acid rain monitor was used to make real-time measurements of conductivity and pH of rainwater within individual storms. The automated measurements were compared with laboratory analyses of a subset of the samples taken. The laboratory measurements tended to overestimate the pH because of temperature induced changes in dissociation and Henry's Law constants affecting ionic compounds in the rainwater. The measurement artefact due to these effects may result in average hydrogen ion concentrations being underestimated by approximately 10 to 15% at UK sites. The greatest systematic discrepancies would be anticipated at highly polluted sites and during low temperature acidic episodes. The concept of a rainwater acid fraction was investigated and found to be useful for quality control and interpretative purposes. The field measurement of conductivity of low ionic strength samples was slightly lower than the corresponding laboratory measurement, possibly caused by limited resolution of the conductivity probe or dissolution of fine particulate material.