Microemulsion (ME) and conventional emulsion (CE) co-polymerization of p-methyl styrene (pMSTY) with methyl methacrylate (MMA) are carried out at 70 • C using n-pentanol (PA) and n-hexanol (HA), respectively, as co-surfactants along with sodium lauryl sulphate (SLS) as surfactant in the reaction media, and potassium persulphate (KPS) as initiator. The co-polymers are characterized by NMR and GPC techniques. The reactivity ratios are evaluated by employing Fineman-Ross (F-R), Kelen-Tüdös (K-T) and Mayo-Lewis integration (M-L-I) methods. The K-T method yields the reactivity ratios 0.68 ± 0.02 (r pMSTY ), 0.40 ± 0.02 (r MMA ), 0.68 ± 0.02 (r pMSTY ) and 0.68 ± 0.02 (r MMA ), respectively, for the ME and CE co-polymerizations of pMSTY and MMA with PA as co-surfactant in the reaction media. The K-T method yields the reactivity ratios 1.09 ± 0.02 (r pMSTY ), 0.37 ± 0.02 (r MMA ) and 0.78 ± 0.02 (r pMSTY ) and 0.54 ± 0.02 (r MMA ), respectively, for the ME and CE co-polymerization of pMSTY and MMA with HA as co-surfactant in the reaction media. Koninklijke Brill NV, Leiden, 2008