Experimental deformation of hot-pressed powders of lizardite serpentinite was carried out to study the effect of the dehydration reaction to olivine + talc + water under controlled pore water pressure. Use of a porous aggregate ensured free movement of pore fluid into or out of the specimen in response to volume changes. The dehydration reaction further increases the porosity of the sample, causing weakening, but progressive pore collapse leads to strain hardening. At low strain rates, a transition to linear–viscous flow was inferred to be caused by the formation of transiently fine-grained olivine in the dehydration reaction. The inability of the rock to support high loads during dehydration at low strain rates means that the production of high-pressure water by dehydration and its subsequent expulsion will favour seismogenic failure in the surrounding rocks not directly involved in the dehydration reaction, rather than the serpentinite itself.