This study develops and tests an experimental method to monitor in situ the dynamic spreading of individual toner particles on model substrates during heating, to simulate on laboratory scale the fusing sub-processes occurring in electrophotographic printing of paper. Real toner particles of cyan, magenta, yellow and black are transformed to perfect spheres by a temperature pre-treatment, then applied to the substrate. either high-energy clean glass or low-energy hydrophobised glass, and heated at rates up to 50 degrees C/min. The subsequent spreading as a function of time (and temperature) is recorded by an optical microscope and CCD camera mounted above the substrate, with the measured drop covering area used to calculate the corresponding toner-substrate-air contact angle. On the hydrophobic substrate the spreading is limited and equal for all four colours, while the Substantially greater spreading on the hydrophilic substrate is accompanied by significant differences between the toner colours. In particular, the cyan and black toners are found to spread to almost twice the extent of the yellow particles. The dynamic spreading behaviour is interpreted in terms of complementary measurements Of Substrate and toner surface energy components and bulk toner rheology, and a simple empirical relation is proposed that fits very well the measurements for all toner and substrate types tested. In particular, the spreading relation is found to be determined only by the toner surface energy and its equilibrium contact angle. with no explicit dependence on toner viscosity. (c) 2005 Elsevier Inc. All rights reserved.