A laser microreactor was used to study six coals to determine the proportion of their inertinite macerals that fused or melted under p.f. combustion conditions, i.e. their percentage of reactive (fusible) inertinite. The method pyrolyses individual monomaceral particles, giving one-to-one correlation between the maceral and the char formed. Photomicrographs of macerals and their chars revealed the wide diversity of char types that derive from inertinite. Classifying these into fused and unfused chars showed that the inertinite divided into fusible and infusible macerals at a particular reflectance value for each coal. Calculations using the whole coal reflectogram gave the percentage of the inertinite that was reactive. This averaged 75% for four Australian coals and 51% for two Laurasian coals, both much larger than previous methods had indicated. Predictions from earlier methods indicated that either the heating rate or the reaction temperature, or both, affect the fusible-infusible boundary and hence the proportion of the inertinite that exhibits reactive (fusible) behaviour.