We report here, for the first time, the dependence of the streaming potential coupling coefficient of two consolidated rock samples (dolomite) in two-phase flow conditions. We used two carbonate core samples characterized by image analysis and petrophysical measurements including porosity, formation factor, resistivity index at 1 kHz, and the critical water saturation determined from capillary pressure curves. In agreement with the results obtained recently by Guichet et al. <2003>, who used an unconsolidated sand, we observed that the relative electrokinetic coupling coefficient scales approximately with the reduced water saturation at capillary pressure equilibrium. This observation is explained with a new model, which makes a clear distinction between Stern and Gouy-Chapman layer contributions to surface electrical conductivity. The streaming potential coupling coefficient becomes dependent on surface conduction in the Gouy-Chapman layer where the pore space is not fully saturated.