During noncatalytic gas-solid reactions, both the chemical composition, and the size and texture of the particle change simultaneously. A new inverse method allows to determine the influence of the reaction parameters (such as temperature) and changes in the specific reaction surface ar ea with conversion. Tt can also establish the range of conversion, where constant reaction conditions are present at the reaction surface. This method is based on the mathematical treatment of experimental results which include the conversion vs, time together with the corresponding reaction parameters. This method consists of several well established mathematical procedures applicable to the specific situation at gas-solid reactions: variable conversion and reaction time are separated and inverted, respectively. The ratio of two reaction rates (of different experiments) at the same conversion provides information about the (only time-dependent) influence of the reaction parameter on the reaction rate. Finally, also the specific reaction surface area as a function of conversion is derived.