Abstract Serpentinite is widely assumed to constitute weak material in subduction zones and to play an essential role for the development of a subduction channel. Information on deformation mechanisms and appropriate rheological models to describe these large-scale flow processes can only be obtained from natural serpentinites exhumed from ancient subduction zones. We examine the microstructural record of HP-metamorphic ( P c. 2±0.5 GPa, T c. 550±50 °C) serpentinites exposed in the Zermatt–Saas zone, Western Alps, using optical and scanning electron microscopy with electron backscatter diffraction (EBSD). The schistose and compositionally layered rocks show pervasive small-scale folding. There is no evidence for any significant deformation by dislocation creep. Instead, the microfabrics including strain shadows and crenulation cleavage indicate that high strain is accumulated by dissolution precipitation creep. In terms of rheology, this suggests Newtonian behaviour and a low viscosity for the long-term flow of serpentinites in deeper levels of subduction zones. This does not preclude dislocation creep and a power law rheology at higher stress levels, as realized at local sites of stress concentration and transient episodes of post-seismic creep.