We have studied the mechanical behavior of serpentinite under conditions favouring dehydration to olivine+talc+water, at temperatures ranging between 300¿ and 600 ¿C and at total pressures from 100 to 300 MPa. Constant stress, stress relaxation and constant displacement rate testing techniques permitted a wide range of deformation rates and paths to be accessed. Effective confining pressure was held constant during the dehydration process by means of a controlled pore water pressure system. Stress supported by the samples was almost independent of deformation rate at temperatures below those required for the onset of the breakdown reaction. At 500¿ and 600 ¿C, however, the strength was linearly dependent on independent of effective confining pressure over the range investigated. Microstructural studies revealed that the weakening was associated with sliding in narrow shear zones containing ultrafine-grained (0.1 μm) olivine produced in the breakdown reaction. The enhanced deformability is interpreted in terms of flow of the olivine in these shear zones by diffusion-accommodated grain boundary sliding. These experimental results suggest that metamorphic transformations affect the deformability of rocks, and demonstrate enhancement of deformability through the transient existence of fine-grained reaction products. The results may also be applicable to the mechanics of deformation processes in oceanic transform faults and give some indication of the degree of weakening that might be expected from grain-size reduction by dynamic recrystallization in plastically deforming rocks. ¿ American Geophysical Union 1988