The importance of adsorption on peat for decreasing NH4+ was estimated in four overland flow areas (OFA) in northern Finland, where peat mining water is purified by conducting it across a natural mire of a given size. The effective cation exchange capacity (ECEC) values of peat in the OFAs ranged from 21.4 to 92.7 meq per 100g, being generally highest in the surface 0–15 cm. It was estimated that about 4.6–5.8% of the peat cation exchan e sites could be occupied by NH4+, corresponding to a NH4+ retention capacity of about 0.18 to 0.77 g kg−1 dry peat. Assuming that the retention was attributable to the 0–15 cm surface peat layer alone, the calculated NH4+ retention potential of OFA peat should have lasted under 6 months with the loadings imposed on the OFAs and the decreases in NH4+ amounts in peat mining water considered here. The actual duration of peat capacity to retain NH4+ in the OFAs has, however, already been shown in this research to be longer, and apparently will be so also in future, probably as a result of two processes: nitrification and subsequent N loss through denitrification, and biological assimilation. Hence, cation exchange capacity of OFA was an important property that contributed to surface water protection against eutrophication by N.