Interspecific variation in plant functional traits is fast becoming popular as a tool for understanding and predicting ecosystem biogeochemistry as dependent on vegetation composition. Leaf pH has recently been shown to be a promising new candidate trait for this purpose. But how robust is leaf pH as a species trait in the face of environmental variation? We hypothesized that inherent interspecific variation in leaf pH should be greater than phenotypic variation of given species in response to soil environments. We tested this hypothesis in a temperate herbaceous flora by growing 23 species experimentally in three soils of contrasting pH (ranging by almost three pH units) and related chemistry. As predicted, there was large and consistent variation in leaf pH among these species, which was robust to the differences between soil types. Indeed both the species rankings and the absolute species values for leaf pH were remarkably constant in comparisons between soil types. The fact that a given species can maintain a leaf pH very different from that of their soil environment, combined with the great interspecific variation in leaf pH, indicates that leaf pH really is largely a species-specific trait. Linked with recent field evidence we suggest that interspecific variation in leaf pH, while easy and cheap to assess, has important predictive power of biogeochemical properties and processes in ecosystems. © 2010 The Authors. Functional Ecology © 2010 British Ecological Society.