The course of emulsion polymerisation is shown to depend on the water‐solubility of the monomers concerned. With insoluble monomers, the mass transfer rate of monomer to polymer particles can sometimes be exceeded by the polymerisation rate, and viscosity within the particles is appreciable throughout the course of the reaction. Thus, surfactant is adsorbed before monomer droplets disappear. With more soluble monomers, mass transfer rates of monomer are high and since the equilibrium concentration of polymer in particles is low the monomer droplets disappear early in the reaction. Consequently, water‐insoluble monomers often conform to Smith‐Ewart behaviour, whereas water‐soluble monomers do not. With water‐soluble monomers the final particle number is proportional to the cube of the initial surfactant concentration.