A detailed study of the adsorption of the n-alkyltrimethylammonium bromides (C(n)TAB) to lysozyme by measurements of the zeta potential (zeta-potential) has been realized as a function of concentration and pH. While at pH 3.2 the zeta-potential of lysozyme remains positive in the presence of surfactants, at pH 7 and 10 the alkylammonium ions affect the zeta-potential causing a change in the neighbourhood of the point of zero charge (pzc) from negative to positive values. From the pzc we have calculated Gibbs energies of adsorption and compared them with Gibbs energies of binding determined by equilibrium dialysis. The results are similar, showing that the zeta-potential technique can be useful in a study of the interactions of proteins with amphiphilic ligands.