The influence of particle residence time distribution on the average conversion rate (or reactivity) of particles undergoing a non-catalytic gas-solid reaction inside a continuously operated fluidized bed reactor is evaluated. A so called ß-factor is defined as the ratio of the actual reactivity in the reactor and the reactivity of a batch of particles that react under similar circumstances and that all have a conversion extent equal to the average conversion extent in the reactor. The ß-factor concept is elaborated for shrinking core conversion behaviour. According to Heesink et al. (1993), three extreme types of conversion behaviour are distinguished: core reaction limitation, product-layer diffusion limitation and grain reaction limitation. For each type of behaviour a mathematical function is derived that expresses ß as function of average particle conversion, maximum attainable conversion (with regard to pore plugging) and a new-defined expansion factor, which is a measure for the expansion (or shrinking) of the reacting solid during conversion. These functions can be easily incorporated in fluidized bed reactor models.