Abstract: Experimental studies of emulsion and suspension polymerisation of styrene at 50℃ are reported. It is apparent that the classical model for Stages I and I of an emulsion polymerisation is not valid for the case of low emulsifier concentrations or for the case of high impeller speeds at low or intermediate emulsifier concentrations. A mathematical model for a batch emulsion polymerisation in an unbaffled reactor is proposed which overcomes this problem by taking into account the effect of the adsorption of the emulsifier onto themonomer droplet surfaces. The model is extended into Stage I by incorporation of the effect of the autoacceleration of the conversion rate through reduced radical translational mobility, i. e. the gel-effect. The model presented in this paper for Stage I of an emulsion polymerisation demands knowledge of the total surface area of the dispersed monomer droplets. An existing relationship between dispersed phase droplet diameter and impeller speed and impeller diameter has been incorporated into the model. The predictions of the particle size, particle number and reaction rate obtained via the current model are in good agreement with the experimental data for Stage I and subsequently for Stage I of the reaction. During Stage I of the emulsion polymerisation the increase in polymer particle viscosity reduces the translational mobility of the radicals with the result that the termination rate constant of radicals increases dramatically. The analogous behaviour in a suspension polymerisation of styrene has been investigated and the following relationship between termination rate constant and the level of monomer conversion has been developed. Kt=exp(A1 + A2Xp+A3Xp2+A4Xp3+A5Xp4) where A1=29.5873, A2=-7.4332, A3=45.8577, A4=-95.9184, A5=47.4095. When this relationship is incorporated into the model for emulsion polymerisation, it allows the determination of the average number of radicals per particle. It is apparent that during Stages I and I this value is close to 0.5 as suggested by the Smith and Ewart case II (instantaneous termination)model but for Stage III the average number of radicals per particle increases with increasing conversion. The predictions of the current model are in good agreement with experimental data of emulsion polymerisation of styrene at 50℃ over most of the conversion range.