AbstractThe deformability of rocks, i.e. the resistance to deformation and the mode of failure, may be strongly influenced by syn‐tectonic metamorphic reactions. Such mechanistic interactions can often be inferred from studies on naturally deformed tectonites or from suitably designed experiments. The localized nature of retrograde reactions sometimes means that the particular interaction can be inferred as a result of the preservation of both the deformed rock and its protolith, but in prograde situations recrystallization at the metamorphic peak tends to destroy microstructural evidence of the deformation mechanism.Syn‐metamorphic mechanistic interaction can be classified according to the effects of: (a) elevated fluid pressure produced during dehydration or decarbonation reactions; (b) transient grain size changes with a possible transition to grain size sensitive flow; (c) effects of changes in pore fluid chemistry and recrystallization on the resistance to plastic flow of silicates (e.g. quartz); (d) enhancement of chemical potential gradients driving diffusive creep; and (e) enhancement of plasticity through the effects of solid phase volume changes. The first three of these effects are expected to be the most important. A soil mechanics approach to the description of the flow of porous rocks provides a basis for understanding deformation during regional prograde reactions, but simple, steady‐state constitutive laws of the type used to describe plastic flow of monomineralic rocks are unlikely to be able to be applied to syn‐metamorphic defo