Mixtures of poly(styrene) latices were used to vary the sample polydispersity for an investigation of polydispersity effects on ellipsometric light scattering (ELS) data. A procedure for high-accuracy ELS measurements is presented and the experimental distinction of coherent and incoherent scattering contributions is demonstrated. The ellipsometric parameters tan(Psi(Q)) and A are solely determined from coherent scattering and represent averaged ensemble properties, independent of the width of the size distribution. The change in polydispersity shows up in the parameter tan(Psi(1)), which is affected by incoherent contributions. The average particle size detected by ELS can be estimated from the intensity-weighted size distribution. A radial local birefringence in the particles has been detected and attributed to stress birefringence due to the action of the interface tension on the particles during their growth.