Electrically induced birefringence experiments were performed on dispersions consisting of sulfate latex nanospheres of two different sizes and charges dispersed in an electrolyte solution, at various ionic strengths. The induced birefringence was found to have an important contribution increasing as a quadratic power law of the volume fraction of the spheres. This shows that interparticle interactions play a role in the observed birefringence. The data were analyzed, using a theory from Hafkenscheid and Vlieger [Physica 75 (1974) 57], in terms of the changes of the interparticle separations in the directions parallel and perpendicular to the applied electric field. (c) 2005 Elsevier Inc. All rights reserved.