Ten mor cores (O‐horizon) were sampled every month during September 1985 to August 1986 (except December) from a 9 m2plot. Each core usually yielded four sliced 1‐cm‐layers in addition to the S‐layer. Each layer was extracted in both distilled water and in 0.01 M CaCl2The spatial pH variation was found to be greater than the seasonal variation in all layers. With few exceptions, pHH2Oand pHCaCl2showed a similar seasonal trend in the entire mor layer. Seasonal alteration of chemical or biological conditions generally influenced pH of all cm‐layers in the same direction. The general seasonal trend of pHH2Oand pHCaCl2was a‐continuous increase during autumn and winter, with a maximum in April, followed by a more drastic decrease and a pHH2Ominimum in June. The minimum pHCaCl2appeared in August. Cation uptake (May‐June) and mineralisation, leaching and alteration of organic acid properties (September‐April, July‐August) were discussed as the dominating factors behind the seasonal pH variation.