The stability of dilute bimodal (diameter: 100 and 200 nm) model latex dispersions is studied as a function of electrolyte concentration and particle number fraction by measuring perikinetic aggregation with dynamic light scattering. A formally correct expression for the effective, doublet stability ratio of a bimodal system is derived that accounts for the difference in the particle size and hence, extends the derivation by Hogg and co-workers [Trans. Faraday Soc. 62 (1966) 1638]. Including the particle size ratio predicts slightly lower stability ratios for polydisperse but chemically similar systems. Stability ratios for binary mixtures of model colloidal latices are extracted from aggregation measurements in the fractal aggregation regime and are compared to predictions based on DLVO calculations of the potential. The results suggest that the composition of the aggregates is dependent on the relative stability of the two components (and consequently, on electrolyte concentration) and is richer in the least stable component. (C) 2003 Elsevier Inc. All rights reserved.