The physical and chemical processes acting in the 'brittle-to-plastic' transition are of great interest for a better understanding of fault rheology. We performed a series of experiments on granitoid gouge material under high confining pressures (Pc = 500-1500 MPa), temperatures (T = 300 degrees C and 500 degrees C) and fast shear strain rates (similar to 1.8 x 10(-4) s(-1)) where the material deforms by semi-brittle flow. Samples deformed at 500 degrees C are systematically weaker than samples deformed at 300 degrees C over the whole examined confining pressure range indicating a non-frictional component of the deformation. All samples develop an S-C' fabrics and deformation localizes in slip zones containing 'amorphous' feldspar material with an intermediate composition (Na, Ca and K-rich). Further, we observe changes in composition of feldspars (enrichment in the albite component) in the highly fragmented - but crystalline - regions with increasing finite shear strain. Our results indicate that mass-transfer processes keep pace with frictional deformation even at high strain rates and together with viscous flow of the 'amorphous' material are responsible for the observed strength difference.