We investigate the thermal consequences of sediment thickening and fluid expulsion in subduction zone accretionary prisms using a model where the rate of fluid expulsion in a uniformly thickening wedge is obtained analytically and the fluid flow and temperature fields are computed numerically. Variations landward across the wedge in the porosity-depth function are included in the model. The most important contribution to the thermal regime arises from the thickening of the wedge, which can reduce the heat flow at the seafloor relative to the deep lithospheric heat flow by up to a few tens of percent. This effect is countered by an opposite but lesser contribution of the heat advected upward by the fluid expelled from the reconsolidating sediments. The total thermal perturbation is sensitive to the prism taper angle, the incoming sediment thickness, and the convergence velocity. Anomalously low seismic velocities observed in a zone several tens of kilometers wide near the toe of some accretionary prisms suggest that there is a delay between the initial thickening of the sediment section and reconsolidation toward equilibrium porosity at depth. The thermal consequences of retarded fluid expulsion are significant, and result in locally depressed seafloor heat flow in this region. For the full observed range of accretion parameters, the rates of fluid expulsion are not sufficient to cause a detectable depth variation in heat flow on a depth scale of less than 1 km. ¿ American Geophysical Union 1993