The surface charge studies were conducted on peat materials at various states of the secondary transformation characterised by W₁ index. A back titration method for the determination of the variable surface charge was applied and distribution of the apparent surface dissociation constants for investigated soils were determined. In this method the peat suspensions were titrated with 0.1 N NaOH in 1 N NaCl solution. For weakly and medium secondary peats positive correlation between quality of the surface charge and the W₁ index was found. The best correlation appeared between the surface charge and the summary content of fulvic and humic acids. However, for proper moorsh material two populations can be distinguished. For all samples very strongly acidic surface sites are dominant. Furthermore, the average apparent surface dissociation constant values were similar for all peat-moorsh soils.