The affinity of the selected heavy metal cations: Mn2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+ and Na+, K+, Mg2+, Ca2+ was determined for the functional groups located on the surface of leached barley middlings, so called Brewer's spent grain (BSG) - a by-product of the brewing industry. A simple method for determining the affinity series, with the use of a conductivity meter and a pH-meter, was described. It was demonstrated that the affinity of cations for the functional groups located on the surface of the BSG increases in the series: Na+ < K+ < Mg2+ < Mn2+ approximate to Zn2+ < Ni2+ approximate to Ca2+ < Cd2+ < Pb2+, and the series was determined by comparing concentrations expressed in gram equivalents, i.e. molar concentrations converted into the individual cation charge. The objective is to assess the possibility to use BSG as a biosorbent in the processes of treatment and post-treatment of waste waters from heavy metals.