We present measurements of the ac susceptibility of sintered yttrium barium copper oxide pellets prepared by a melt processed solid state reaction method, with different amounts of ZnO added. The measurements were performed versus temperature T for several different ac magnetic field amplitudes H-ac in the presence of static bias magnetic fields H-b directed along H-ac. For all our samples, for each choice of H-ac, the imaginary part of the ac susceptibility versus T traces a peak, denoted by chi"(max), centered at a temperature denoted by T-max.We also find that plots of the values of chi(")(max) vs H-ac and of chi(max) vs T-max generate curves that display a valley. Calculations of hysteresis losses, exploiting a critical-state model where the critical current density j(c)=alpha(T)/B-n, reproduce the experimental data quite well. (c) 2006 American Institute of Physics.