The DLVO theory offers conceptual problems now that it appears to be established that double layer potentials are low at the critical coagulation concentration in colloids which follow the Schulze-Hardy rule, and that the predicted relation between the rate of slow coagulation and the particle size is not found experimentally. In this paper we explore the hypothesis that competition between aggregation and repeptization determines the net rate of slow coagulation, and that aggregates become progressively more irreversible after their formation. A theory of this kind could account for many experimental observations. It predicts that the double layer potential φδ at Cc increases with the counterion valency and that there is some critical potential φ0δ below which coagulation can begin.