Measurements of the intrinsic reactivity to oxygen of chars are increasingly being sought as an indicator of the combustion potential of fuels, particularly with respect to residual char bum-out. A TG-based method is described which is suitable for both routine industrial use and for research studies. Ramp heating, typically at 15 K/min, offers the significant advantages of allowing the entire sample to be assessed, requiring only a short time (approximately 1 h) to complete and being able to use the same conditions to test almost any sample. Reduced oxygen concentrations (6.3% O-2 v/v) minimise uncontrolled exothermic heating of the sample; this is also comparable to oxygen levels encountered in the later stages of pulverised fuel (PF) combustion. Data processing methods, made possible by developments in computer hardware and software, have been developed to give fundamental, quantitative measurements of char reactivity, allowing direct comparison of the overall reactivities of different chars. Information on the heterogeneity of chars can also be derived. Typical results for a range of coal chars and comparison with other TG-based char characterisation methods are presented. Reactivity values measured on two TG instruments with different geometries, both of which operated using the methods developed, have been compared and found to be in agreement.