The kinetics of the free-radical polymerization of methyl methacrylate at 60 °C in the presence of large amounts of n-dodecanethiol (DDM) were investigated as a test at short chain lengths of our recently proposed composite termination model [Smith GB, Russell GT, Heuts JPA. Macromol Theory Simul 2003;12:299]. The experimental data were found to show an unexpected and therefore very interesting trend: at high concentrations of DDM, the rate actually increases as chain lengths become smaller. No termination model can explain this unusual behaviour. However it was found that these results are very successfully described when, in addition to our composite termination model, chain length dependent propagation is included in modelling of the kinetics. These findings have important ramifications for all kinetic studies involving short polymeric radicals, including in low conversion living radical polymerization systems and studies determining the chain length dependence of the termination rate coefficient.