We formulate a model for the peptization of titania nanocolloids. The model assumes simultaneous agglomeration/deagglomeration with size-independent rate constants. It predicts that the evolution of the particle size exhibits first-order kinetics and that the final particle size scales as R similar to C-1d/f, where C is the mass concentration of the colloid and d(f) is the fractal dimension. These predictions are tested experimentally. We find that the model provides a quantitative and consistent description of the peptization process over a wide range of experimental variables and that it allows the determination of the aggregation and deaggregation rate constants from the time evolution of the average size.