Understanding the thermal structure of the lithosphere in subduction zone environments is a key element in the realization of the source of deep focus earthquakes and other geodynamic processes. Model predictions of the thermal structure of subducting slabs are dependent upon a priori knowledge of how thermal conductivity varies within the slab. Using recent results for the variation of thermal conductivity as a function of temperature, pressure, and gross mineralogy, we investigate how models that utilize a realistic model for variable conductivity compare with canonical models for constant conductivity. Results suggest that jumps in thermal conductivity across the phase boundaries that arise due to transformations to &bgr; and &ggr;-spinel phases lead to old, fast, steeply dipping slabs being considerably warmer than previously thought. Although these results do not rule out the possibility of a metastable wedge source of deep focus earthquakes, they do imply the likelihood of metastable olivine being found at depth is less than once thought. ¿ 1999 American Geophysical Union