Anomalies during precipitation fractionation of PVC from THF solution with H20 as nonsolvent were investigated by using GPC and solution infrared techniques. The data indicate that separation during the fractionation takes place by both stereoregularity and molecular weight because of the formation of molecular aggregates during the precipitation and collection of fractions. The separation by stereoregularity dominates at the beginning of the fractionation. The aggregates formed at this point are comprised of highly syndiotactic molecular segments which tend to precipitate before segments of lower stereo-regularity but higher molecular weight. For the first few fractions, molecular weights recorded may be seriously overestimated unless care is taken to disperse the aggregates by appropriate heating of the solutions used for GPC characterizations. After 25-30% of the polymer precipitates, the fractionation becomes independent of stereoregularity and proceeds according to molecular weight. While the tendency for aggregation may be great for highly syndiotactic and/or high molecular weight chain segments, the data for low molecular weight fractions having low stereoregularities also show evidence of molecular aggregation. This suggests that factors such as syndiotactic sequence length and degree of branching may be quite important in determining aggregation tendencies. Typical GPC chromatograms indicate that the fractions which contain highly aggregated molecules have bimodal GPC curves. Complete resolution of the aggregate peak from the soluble portion can be achieved by operating the GPC in the recycle mode. Aggregation may not be apparent from GPC curves obtained for PVC whole polymers, particularly atactic commercial grade samples. Among samples with similar average molecular weights anomalies in fractionation increase with polymer stereoregularity. Because separation by stereoregularity dominates at the beginning of a fractionation a “molecular weight inversion” may occur where successive fractions have higher molecular weights than the initial fraction. The syndiotacticity (S values) of the fractions vary indicating that each polymer has a unique syndiotacticity distribution. The observations reported have been verified in many fractionations with several different PVC whole polymers.