The factors determining fuse quality in hot-roll toner fixing, namely, toner and paper surface properties and machine conditions were examined. The SEM observation of the fixing states and the tape peeling test failure states showed the direct relation between the fuse quality and the toner fixing states. A physical fusing model was employed to explain the change in the shape of toner particles during the fixing process. The non-uniform fixing states were categorized into three levels. The first group comprised unmelted or only marginally melted toner particles which were weakly adhering to a paper surface and thus in a very early stage in the fusing model. The second group was composed of several combined toner particles in the middle point of the fusing model. The third group was made up of well sintered and spread toner particles called islands. These were strongly adhered to a paper surface and did not peel off by the tape test. The effect of heat and pressure on the fuse quality was to put forward the non-uniform fixing states toward the end of the fusing model giving rise to a better fuse quality. Toner surface tension measurements by a sink-float method indicated a broad distribution from 23 to 38 dyne/cm. This result was supported by ESCA data showing a clear difference in surface chemical composition of nitrogen between the high and low surface tension components of the toner. The broad range of toner surface tension could be one of the causes of the non-uniform fixing states. Paper properties, roughness and spreading also played key roles in toner fixing. The better fuse quality with the smoother paper surface was explained by a proposed model developed from paper surface roughness analysis data obtained by a traversing stylus and the knowledge of toner particle size by SEM. Differences in spreading ability of paper were responsible for observed differences in the fuse quality of paper samples at the same paper roughness. Greater spreading led to better fuse quality by obtaining more intimate contact of toner melt and paper. ESCA analyses of paper and toner surfaces suggest that spreading followed by adhesion is due to the interaction between paper and toner polar groups. / M.S.