Acidity detns. were made monthly for 5 yrs. on the upper and lower layers of the soil in 7 stations representing 3 types of woodland soil. Statistically significant annual and monthly variations in acidity were found in all 3 types. The annual variations can be related to variations in total annual rainfall. The acidity of gravels is increased but that of clays and loams decreased during years of low rainfall. The monthly variations are in part related to large monthly variations in rainfall, but there is some evidence also of an annual rhythm in acidity. The acidity changes are ascribed to changes in micro-organic activity and, in the heavy soils, to the capillary ascent of base-rich water during dry periods.