The interaction between the matrix of a porous material and a small quantity of a liquid located in its porous space induces intense polarisation phenomena and enhances the charge transport. The temperature dependence of the corresponding relaxation time was reported recently by employing the high-resolution thermally stimulated depolarisation current spectroscopy. The present work includes d.c. conductivity and a.c. capacitance measurements under pressure up to 0.3 GPa in two porous silicate materials: sandstone with 0.26 wt.% and pelite with 0.07 wt.%. The water was adsorbed from the atmospheric humidity. Electrical impedance measurements are used to monitor the effects of confining pressure on the matrix-liquid interaction. Positive activation volumes are computed from the observed variations in the d.c. conductivity of the samples with pressure. The pressure derivatives of the dielectric constant at 10(4) Hz were also obtained. The effect of pressure on the effective relaxation time that determines the humidity-related polarisation processes was estimated.