Seventy-four soil profiles in old spruce, beech and oak stands at Tönnersjöheden Experimental Forest were sampled both in 1927 and 1982–83. Each new profile was located within about one metre of the sampling point from 1927. Samples were collected from the A0, A2, B and C horizons down to a depth of 70 cm in mineral soil. Both in 1927 and 1982–83, pH in the soil samples was measured potentiometrically (quinhydrone electrode). The new pH-measurements were also made with a glass electrode. Today, lower pH-values were obtained in all sampled soil horizons compared with 1927. The decline in pH is 0.3–0.9 units in the humus layer and 0.3–0.7 units in the mineral soil.