THE purpose of this note is to call attention to the fact that usual methods of measuring soil pH1 seriously underestimate the pH of very acid soils, such as those of coal mine spoils2, cat clays3 and solfataras4, in which acidity is due to the presence of free sulphuric acid. Standard methods for measuring soil pH involve making a slurry of soil with (usually) distilled water at a 1 : 2, 1 : 5, or 1 : 10 dilution, and reading the pH of this slurry. Implicit in this procedure is the assumption that the soil is buffered and hence that pH does not change with dilution. This assumption is erroneous in the case of the sulphuric acid soils.