This paper investigates the enthalpy of solid phase transition of isotactic polypropylene from the mesomorphic phase to the monoclinic crystal form using DSC. iPP fibres with either monoclinic crystal form or mesomorphic phase were obtained by varying the conditions of melt spinning XRD and mechanical tests were used to confirm the crystal forms. It is found that conventional DSC techniques fail to detect the transition enthalpy; a technique using silicon oil as the thermal medium was then employed to determine the transition enthalpy, which is of 3% the melting enthalpy for crystallites with monoclinic form, or 6.1 J/g on 100% crystallinity basis. (c) 2005 Elsevier Ltd. All rights reserved.