To quantify the effects of surface conduction behind the slip plane on the electrokinetic transport properties of colloids, we have extended the thin-double-layer theory of Fixman for dilute sols of spherical particles. The computations show that it causes the static conductivity and the low-frequency dielectric response of the sol to increase and the mobility of the particles to decrease. Hence, the occurrence of surface conduction significantly changes the relationships between the different electrokinetic properties. Therefore, for a rigorous interpretation of experimental results taking possible effects of surface conduction adequately into account, it is imperative to collect data with more than one electrokinetic technique. For polystyrene latices, a comparison between theory and experiment has been made. The mobility and static conductivity data can be well reconciled if surface conduction is accounted for. The corresponding estimates of surface charge densities agree very well with values obtained by independent measurements. However, the extreme dielectric behaviour of latex colloids cannot be attributed to surface conduction.