For dilute solutions of poly(methyl methacrylate) in isoamyl acetate with the molecular weight M-w = 2.35 X 10(6), the phase separation process was observed as an aggregation process of polymer chains by light-scattering measurements. The aggregation process was measured for a period of hours at four polymer concentrations and at about 15 K below the phase separation temperature. The light-scattering data analysis by Guinier plot yielded the average molecular weight [M](w) and radius [R-2](z)(1/2) for polymer aggregates as a function of time t and revealed the exponential growth of [M](w)similar to e(gt) and [R-2](z)similar to e(ht). The coefficients g and h were proportional to the polymer concentration. A shape of the observed scattering function was independent of the concentration and time. The fractal dimension for [M](w)similar to[R-2](z)(D/2) was determined to be D = 2.86+/-0.03. These characteristic features of the polymer aggregation were represented by the Smoluchowski equation for cluster-cluster aggregation with the collision kernel (i + j) for i mer and j mer. The observed scattering function and fractal dimension were analyzed by the Smoluchowski equation with the assumed value D = 3 for monodisperse clusters.