The cationic polymerization of p-methoxystyrene (pMOS) in miniemulsion in the presence of dodecylbenzenesulfonic acid (DBSA) was studied. DBSA acts as both protonic initiator and surfactant (INISURF). The recipe was first optimized to generate stable latexes from a miniemulsion, i.e., droplets/ particles of constant size throughout the polymerization. The miniemulsion polymerization process was highly reproducible, quite fast at high temperatures (100% conversion in 8 h at 60 °C) and applicable even at high monomer content (typically 40 wt %). Poly(pMOS) of small average molar masses (≈1000 g mol -1 ) and controlled functionality were synthesized. The sharp increase in molar masses at low conversion was indebted to a decrease of water content at the interface through a cosurfactant effect. Above 20% conversion, the chain length limitation, also observed in previous ionic polymerizations in emulsion studies and referred to as critical DP, was confirmed by modeling simple kinetics. At final conversion, polymer degradation occurred as revealed by SEC, MALDI-TOF, and 1 H NMR. The particles quickly coalesced in the absence of a cosurfactant.