The spontaneous (thermal) initiation rate (R-i,R-th; no added initiator) in radical polymerization of styrene in aqueous miniemulsion at 110 and 125 degrees C with sodium dodecylbenzenesulfonate or poly(vinyl alcohol) as surfactants (colloidal stabilizers) has been estimated using a novel approach based on the total number of chains. In qualitative agreement with previous work at lower temperatures, R-i,R-th was found to be 3.1-15.1 times greater than that in bulk. According to the activation energy and conversion dependence of R-i.th, the radical generation mechanism differs from that in bulk. The experimental evidence is consistent with the enhanced R-i,R-th in ininiernulsion being related to the oil-water interface, with radical generation in the aqueous phase playing a negligible role. The implications with regards to nitroxide-mediated radical polymerization in aqueous dispersed systems are discussed. (C) 2008 Elsevier Ltd. All rights reserved.