The modification of porous PVC particles by an in-situ stabilizer-free polymerization/crosslinking of a monomer/crosslinker/peroxide solution absorbed within the PVC particles is presented. The modifying crosslinked polymers are polystyrene (PS) crosslinked with DVB (divinyl benzene), polymethyl methacrylate (PMMA) crosslinked with ethylene glycol dimethacrylate (EGDMA), and styrene-MMA copolymer crosslinked with DVB. The modified PVC particles characterization includes polymerization yield, non-extractables, C-13 solid-state CPMAS NMR, porosity measurements and also morphology and dynamic mechanical behavior (DMTA). The levels of non-extractable fractions found and C-13 solid-state CPMAS NMR results are indicative of low chemical interaction in the semi-IPN PVC particles. Particle porosity levels and SEM observations indicate that styrene and MMA mainly polymerize within the PVC particles' bulk and just small amounts in the pores. MMA polymerization in the PVC pores is as crusts covering the PVC pore surfaces, whereas styrene polymerization in the PVC pores is by filling the pores. Dynamic mechanical studies show that tandelta and the storage modulus curves are influenced by the incorporation of PS and XPS but not by the incorporation of PMMA and XPMMA.