Two main theories exist to explain the manifestation of electrical potential changes during rock deformation; (i) the electrokinetic effect due to fluid flow within a porous medium. This theory, derived from colloidal science relates the magnitude of this potential anomaly to the permeability and electrical conductivity of the medium. (ii) Secondly the piezoelectric effect in which electric dipoles are created due to the application of stress on specific crystals e.g. quartz. Here we present laboratory measurements of electrical potential and acotsstic emission from sandstone (75% quartz) and basalt (0% quartz) during triaxial deformation. An increase in electric potential occurs during deformation for both dry and saturated sandstone and basalt. The relationship between electrical potential change and axial stress is linear for dry sandstone and dry basalt as expected from piezoclectric theory. Non- linearity is observed for saturated sandstone and basalt and indicates the effect of the electrokinetic effect. The effect of strain induced craclcing, located in 3D, on each of the two aforementioned theories is discussed.