Polystyrene (PS) and poly(methyl methacrylate) (PMMA) samples produced by the bulk homopolymerization of styrene and methyl methacrylate with a tetra-functional peroxide initiator (JWEB50) are characterized in detail by various solution and rheological methods. For comparison purposes, "linear" PS and PMMA samples were produced under similar conditions with a monofunctional initiator (TBEC). The four sample types were characterized by size exclusion chromatography (SEC) setups to determine molecular weight, radius of gyration, and intrinsic viscosity distributions. Contraction factors were calculated and indicated evidence of branching for polystyrene produced with JWEB50 while no such effects were observed with PMMA. The rheological behavior of the samples was subsequently investigated by performing oscillatory shear and creep experiments. Compared to the "linear" material, samples produced with JWEB50 exhibited a reduction in zero-shear viscosity that was attributed to long-chain branching. Retardation spectra were calculated based on creep data and converted to dynamic compliances that were then combined with the oscillatory data. This provided master curves spanning a much wider frequency range than could be obtained experimentally. Examination of various viscoelastic functions showed evidence of long-chain branching for both polystyrene and poly(methyl methacrylate) samples produced with JWEB50. (c) 2006 Wiley Periodicals, Inc.