Grain-scale deformation processes during sliding on simulated faults, and in the presence of reactive pore fluids, are examined with in situ, see-through experiments using polycrystalline biphenyl aggregates at 60 °C as a rock analogue. Reactive pore fluids were water-alcohol-vapor mixtures and bulk shear strain rates were 5.8 × 10−6 s−1. Under these conditions, slip at displacement rates around 0.01 µm/s on shear fractures involves a mixture of cataclasis and frictional sliding, together with dissolution and precipitation processes. Simultaneously, dislocation flow processes accommodate distributed shearing away from slip surfaces. The evolution of fault porosity is controlled by a dynamic competition between crack growth and opening and crystal growth into cracks and pore spaces. During progressive sliding, growing crystals and other asperities on opposing fracture walls interact by fracturing, rotation, grain translation, and dissolution and precipitation. This multimechanism behavior in rock analogue materials provides insights about the mechanical behavior of faults in the presence of hydrothermal fluids at seismogenic depths. Models of the strength and slip behavior of natural faults may thus need to consider multimechanism behavior during frictional sliding.