The microencapsulation of phase change materials (PCMs) by a suspension copolymerisation of styrene (St) and methyl methacrylate (MMA) has been studied. The influence of the monomers/paraffin and the MMA/St mass ratio on the encapsulation process and the physical properties of the resulting microcapsules has been studied. The thermal properties, morphology, chemical composition and particle size distribution of the microcapsules were characterised by differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), environmental scanning electron microscopy (ESEM), optical microscopy (OM), Fourier transform infrared spectroscopy (FT-IR) and low angle laser light scattering (LALLS). It was found that the MMA/St mass ratio has a marked influence on the polymerisation rate, affecting the conversion and the time at which the identity point is reached. As the amount of MMA is increased, the reaction time and the mean particle size decrease. On the other hand, paraffin was not encapsulated when the monomers/paraffin mass ratio was lower than 3.0 due to the shortage of copolymer. On the basis of the experimental results, an MMA/St mass ratio of 4.0 and monomers/paraffin ratio of 3.0 were established as the best conditions to produce this kind of microcapsule. The higher reactivity and solubility of the methyl methacrylate in water, when compared to styrene, favours paraffin microencapsulation-as shown by the average storage energy capacity of the microcapsules obtained using this copolymer. Furthermore, TGA analysis shows that the thermal stabilities of both the virgin copolymer and the microcapsules containing PCMs are similar when P(St-co-MMA) is used as the shell material. (C) 2009 Elsevier B. V. All rights reserved.