Electron spin resonance (e.s.r.) spectroscopy should be useful for the measurement of propagation rate coefficients (k(p)) in free radical polymerisations, since the propagating radical concentration can be directly observed. However, in previous studies, k(p) values have generally been inconsistent, and disagreed with data from other accepted experimental techniques, such as pulsed laser polymerisation. This study shows that for styrene, consistency can be obtained, but only when experimental conditions are selected such that polymer of sufficiently high molecular weight is formed; otherwise incorrect rate constants may be measured. The measured Arrhenius parameters were in good agreement with IUPAC accepted data. The values at 343 K for both thermally and photochemically initiated experiments were found to be consistent, suggesting that photoinitiated systems may polymerise in the ground state. Some previously inconsistent data could be explained in terms of selection of experimental conditions.