Small-angle neutron scattering (SANS) studies have been carried out on a binary mixture of polystyrene-h(8) particles (A, radius 168 Angstrom) and polystyrene-d(8) particles (B, radius 510 Angstrom). Two dispersions were prepared at a total volume fraction of ca. 0.03 and with number ratios N-A/N-B of 9 and 15. The electrolyte content of these dispersions was 6 x 10(-5) mol dm(-3) sodium chloride. Experiments were carried out at 3 contrasts using H2O, 75% D2O, and D2O so that the data obtained enabled evaluation of the three partial structure factors, S(Q)(AA), S(Q)(BB), and S(Q)(AB). Prior to examining the mixtures, separate SANS experiments were carried out to characterize the particles and to obtain the structure factors for the individual latices, S(Q)(A) and S(Q)(B). This enabled a comparison to be made of the single-component dispersions and the binary mixtures. The results indicate that the larger particles retain their structure in the mixture but that the structure of the smaller particles is distorted with evidence of long-range correlations resulting in "weak" cluster formation. The variation in the partial structure factors is consistent with the calculation that the strongest electrostatic interaction is between the larger B particles and the weakest between the smaller A particles.