In order to study the microstructure development across the brittle-viscous transition and to derive the corresponding flow laws, we performed shear experiments on quartz gouge in a Griggs-type deformation apparatus. The starting material is a crushed quartz single crystal (sieved grain size <100 μm) with 0.2 wt% water added. The experiments were conducted at temperatures between 500 ° C and 1000 ° C at confining pressures of 0.5 GPa, 1.0 GPa or 1.5 GPa. Four strain-rate-stepping experiments were conducted at strain rates between ∼2.5 x 10-6 s-1 and ∼2.5 x 10-4 s-1. Other experiments were conducted at constant strain rates of ∼2.5 x 10-6 s-1, ∼2.5 x 10-5 s-1, ∼2.5 x 10-4 s-1 and ∼2.5 x 10-3 s-1. At high confining pressure, the strength of the samples decreases with increasing temperature for all strain rates. The largest decrease occurred between 650 ° C and 700 ° C at shear strain rates of ∼2.5 x 10-5 s-1. At the same time, the pressure dependence of strength is positive for T ≤ 650 ° C while an inverse pressure dependence is observed at T > 650 ° C. For T < 700 ° C, the friction coefficient decreases slightly with increasing temperature, from 700-1000 ° C it shows a strong temperature dependence. Between 650 ° C and 700 ° C at shear strain rates of ∼2.5 x 10-5 s-1 a change in the deformation process occurs from one dominated by cataclastic flow to one dominated by crystal plasticity. The microstructure reveals a less abrupt transition in terms of operating processes, because brittle and viscous processes are equally active around 650 ° C. With increasing temperature the volume fraction of recrystallised grains increases, and at 900 ° C - 1000 ° C recrystallisation is nearly complete at strains of γ ∼ 3. The crystallographic preferred orientation of the c-axis evolves from a random distribution at low temperatures towards two peripheral maxima at intermediate temperatures. At high temperatures the c-axis show a single Y-maximum. At high temperature, the stress exponent is n = 2.1 ± 0.2. The activation energy Q is 193 ± 12 kJ/mol at strain rates of 10-5 s-1, at faster strain rates the activation energy drops down to Q = 119 ± 12 kJ/mol. This small stress exponent at high temperatures indicates a combination of deformation processes (diffusion in very fine grained material and dislocation creep in coarser grained material). At lower temperatures the n-value is significantly higher (n ∼ 8) indicating the beginning of the power-law-breakdown. Interestingly, the mechanical data indicate a sharp transition between brittle- and viscous-dominated regimes around 650 ° C to 700 ° C, while the microstructure reveals a smoother transition over a wider temperature range. The relatively low stress exponent of n ∼ 2 clearly suggests the activation of diffusion creep deformation processes.