A study of the potentiometric titration and metal ion binding properties of peat is reported. Peat is found to behave as a typical polyelectrolyte system which, especially at high pH's, can be very highly charged. One difference between peat and commonly investigated polyelectrolyte systems (like poly(acrylate)) is that a very broad spectrum of pKa values is required for the description of the titration characteristics of peat. The prime reason for this is assigned to the presence of humic acids, formed during decomposition of plant parts. The humic acids constitute a rather badly defined group of polyelectrolytes built up inter alia of carboxylic, phenolic and sugar groups. The presence of different length segments of conjugated double bonds provides conditions for light absorption at a broad range of wave-lengths, hence the black or brown colour of these compounds, as well as the stabilisation of resonance structures including carboxylate and phenolate groups. Because of large differences in double-bond segment lengths, a broad range ofpKa values results.