We have investigated the magnetic behaviour of a sintered YBCO sample by ac susceptibility measurements in the presence of a static bias do magnetic field. Calculations of the imaginary part of the ac susceptibility or hysteresis losses have been performed to account for the measured ac loss valley. Our calculations based on the critical state model with j(c)=alpha(T)/B" reproduce the experimental data quite well. We also explored the effect of temperature exponent in the pinning strength parameter on the ac loss valley behaviour. (c) 2005 Elsevier B.V. All rights reserved.