The kinetics and mechanism of free radical polymerisation of acrylonitrile (AN), using potassium peroxydisulphate as initiator in the presence of propiophenonebenzyldimethylammonium chloride as phase transfer catalyst were studied. The reactions were carried out under inert and unstirred conditions at constant temperature of 60 degrees C in cyclohexanone/water biphase media. The dependence of the rate of polymerisation on various experimental conditions such as different concentrations of monomer, initiator, phase transfer catalyst (PTC) and different ionic strength, temperature and volume fraction of aqueous phase is studied. The order with respect to monomer, initiator and the phase transfer catalyst is found to be 2, 0.5 and 0.5 respectively. The rate of polymerisation (R-p) is independent of ionic strength and pH. However, an increase in the polarity of solvent and volume fraction of aqueous phase has slightly increased the R-p value. Based on the results obtained, a mechanism has been proposed for the polymerisation reaction.