We consider the zeta-potential and the effective charge of spherical polyelectrolyte brushes (SPBs) in aqueous solution in the presence of trivalent europium ions. The SPB consists of a polystyrene core of ca. 250 nm diameter onto which long chains of the strong polyelectrolyte poly(styrene sulfonate) are grafted (contour length: 82 nm). At low concentration of EuCl3 the chains are stretched to nearly full length. If the concentration of the trivalent ions is raised, the surface layer of the polyelectrolyte chains collapses. The zeta-potential of the SPB is calculated from the electrophoretic mobilities measured at different concentrations of EuCl3. At the collapse, zeta decreases by the partial neutralization of the charges by the trivalent ions. The experimental zeta-potential thus obtained agrees with the theoretical surface potential Psi(theo) calculated for the effective shear plane by a variational free energy model of the SPB. (C) 2009 Elsevier Inc. All rights reserved.