In this study, the influence of different fluids on the transverse compressibility behaviour of technical reinforcing fabrics was investigated. Specifically, a glass fibre woven and a non-crimp fabric with comparable fibre areal weight, were studied in a novel test-rig with in-situ impregnation of the samples under transverse load. Four test fluids with varying viscosity and polarity, respectively, were chosen to analyse the influence of fluid viscosity and polarity on the relaxation behaviour. In addition, silicone oil with four different levels of viscosity across three orders of magnitude was used to isolate the influence of fluid viscosity, as no significant difference in polarity was observed. A qualitative analysis showed that the fabrics reacted significantly different to the impregnation, as with lower flexibility and increasing viscosity, hydrostatic effects in the compressed area counteracted a lubricating effect. However, only in the fabric with lower fibre bundle flexibility a slight to significant relative pressure relaxation was measured only as a result of fluid polarity. In addition, the viscosity of the fluid had no significant effect on the relative relaxation at the same polarity, but with increasing viscosity the time to reach pressure equilibrium increased significantly.