Humic substances (HS) were extracted from the soil and water of an upland catchment and characterised with respect to molecular weight, C, H, O, N and S contents and ash content. Three samples were obtained: peat humic acid, aquatic humic acid and aquatic fulvic acid. The weight average molecular weights were found to increase from 2400 for aquatic fulvic acid, to 6300 for aquatic humic acid and to 16500 for peat humic acid. Acid-base titrations were performed to assess the proton binding behaviour at different ionic strengths in NaCl and in the presence of important cation competitors (calcium and magnesium). The results from the titrations were fitted to a recently developed discrete-site/electrostatic model of cation-humic binding. Model parameters showed systematic trends with molecular weight and good agreement with parameters derived from the analysis of literature data. In the order peat humic, aquatic humic and aquatic fulvic acid, the milli-equivalents of carboxylic acid groups per gram of HS (nA) increased. In the same order both the negative log10 of the mid-range intrinsic proton dissociation constant for the strong acid groups (pKA) and the empirical constant, P, which accounts for electrostatic effects, decreased.