Capsules containing paraffin as phase change core were in situ synthesized by the absorption and polymerization of urea-formaldehyde prepolymer onto the core in the presence of hydrolyzed styrene-maleic anhydride copolymer as emulsifier in aqueous phase. The particle size and shell content were controlled. A series of capsules in the form of powder have been synthesized with high thermal enthalpy capacity due to the high crystallization of the core paraffin. They were stable against coalescence after heating/cooling cycles. The capsules can find potential applications such as in energy conservation, functional heat fluid and heat protection materials. (C) 2008 Elsevier Ltd. All rights reserved.