Interaction between ceramic compacts and the encapsulation glass during the HIP process has been studied in a model system of silicon nitride and borosilicate glass. Attention has been focused on what happens when the pressure is first applied in the HIP-cycle, i.e. between about 1200 and 1500°C. At this stage the pore system of the ceramic green body is still rather unaffected by sintering. The model system was characterised to evaluate a possible viscous flow of glass into the green body. Two glass compositions, one with high and one with low viscosity, were used, measurements being made of their viscosity and their contact angle on the nitride. Applying Darcy's law it was predicted that the encapsulation glass with the lowest viscosity should penetrate about 1200 microns into the still open pore structure at 1450°C, but this was not observed experimentally. In the calculations no chemical reactions were assumed to take place. However, increases in hardness of heat-treated mixture of glass and silicon nitride powder indicates that nitrogen dissolves in the glass. It is known that nitrogen increases the viscosity of the glass and this would result in a more limited glass intrusion. After HIP the surface region of the dense ceramic exhibited a phase composition gradient of silicon oxynitride, down to approximately 100-200 microns into the bulk