A new look at mass transfer in olefin polymerization shows that neglecting the possibility of convection inside growing polymer particles can lead to conflicting results in certain cases. Direct application of the complete mass flux equation leads to an expression that includes both molar diffusion and a molar average velocity. In cases where a highly reactive species, usually ethene, is present in a concentrated state and is polymerized with a less reactive comonomer, this second monomer can be entrained by the convective currents and accumulate inside the particle. This effect, while not pronounced for small catalyst particles of relatively low activity, increases in importance as the size of the fresh catalyst. It is also shown that although convective flow can lead to the accumulation of an inert species inside the particles, there seems to be no significant rate-reducing effect of accumulation under common industrial operating conditions.