The coke reactivity index (CRI) measurement determined with the ASTM standard is affected by both the chemical and physical properties of the coke. The contribution made by each of these factors to the measurement result cannot be easily distinguished and quantified. A new thermogravimetric analysis (TGA) technique was developed to provide further information about the chemical behaviour of the coke to facilitate an improved interpretation of the CRI measurement. The development of the technique takes into consideration the potential change in reaction rate controlling mechanism in TGA equipment. It makes use of the shrinking core model to identify the reaction rate control regime and to extract reaction rate parameters from experimental measurement. A rigorous data processing procedure was developed to ensure consistent interpretation of the measurement data while repeatability was established through measurement repetition. The sensitivity of common variable factors between different TGA apparatus on measurement results was also examined. It was demonstrated that the measurement results of this new technique are highly repeatable and reproducible.