The characterization of a polymer mixture made of individual linear chains and clusters is often hindered by the presence of a small amount of high molecular weight clusters. Here, an analytical method combining both static and dynamic laser light scattering (LLS) was utilized to characterize such polymer mixtures. The LLS method was carefully verified by using a set of polymer mixtures made of two polystyrenes, in which the two polystyrenes have a very large difference in their molecular weights and the higher molecular weight polystyrene was used as an imitation of the clusters. We have successfully applied this LLS method in the study of PES-C (phenolphthalein poly(ether sulfone)), a newly developed high-performance thermoplastic resin. The PES-C sample contains about 6% high molecular weight-clusters by weight. The large clusters can be removed by a filter with a smaller pore size. The average molecular weight of the clusters is similar to 15 times higher than that of the linear PES-C chains. The PES-C linear chain has a random coil conformation in chloroform at room temperature, while the structure of the clusters seems to be more compact. This LLS method should also be very useful in the study of aggregation or association in a polymer or colloid system as long as the large "clusters" are reasonably stable in time.