Analysis of published experimental data on monomeric radical diffusion in the emulsion polymerisation of styrene shows that it can be quantitatively described equally well by non-equilibrium diffusion from particles, where all parameters are derived from properties of the discrete phase, or by steady-state diffusion where all parameters are derived from properties of the continuous phase. The non-equilibrium model better describes an observed experimental trend to a reduced desorption rate coefficient at higher weight fraction of polymer in the particles. The theoretical upper bound of the non-equilibrium model is also higher than the theoretical upper bound of the steady-state model allowing fits to experimental data which must be discarded as anomalous in the continuous phase model.