This article presents the SEC analysis of branched polyisobutylene PIB and polystyrene PS with high molecular weight and broad multimodal molecular weight distribution. Both polymers were synthesized using an inimer technique, which results in long-chain branched polymers with statistical branching and broad multimodal distributions. Using high resolution multidetector Size Exclusion Chromatography SEC the polymers were analyzed based on three branching factors: g = (R(g)(z,br)(2)/R(g)(z)(2),(lin))(Mw); h = (< R(h)>(z,br)/< R(h)>(z),(lin))(Mw); and rho = (R(g)(2) (1/2)/< R(h)>(z)). It is generally accepted that for monodisperse branched polymers g and h < 1. In the case of our polydisperse PIB and PS, it was seen that g and h > 1, and rho increases with molar mass and the number of chain ends as predicted earlier. The multidetector SEC system allowed for the separation of branching and polydispersity, reported here for the first time experimentally. The g parameter as a function of DP(i) was compared to the theory developed by Zimm and Stockmayer. The plots followed a similar trend, but were shifted by a factor related to the average chain length between branching points. The rho parameter decreased with increasing DP(i), as predicted theoretically by Kajiwara. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 70-79, 2012