The phenomenon of shear-induced aggregation has been investigated in dilute solutions of high molar mass polystyrene standards in phthalic acid esters by light scattering and simultaneous rheometrical measurements. An optical-rheological correlation is found which can easily be understood by simple molecular kinetic models. Systematic examination of the polymer solutions, varying temperature, concentration, and molar mass, shows a dependence of the aggregation on these parameters which fits thermodynamic models.