Experiments have been performed to measure the rates of oil shale pyrolysis. The measurements have been used to develop a model of the reaction rates that takes into account resistance to the evolution of product. In the experiment, samples of shale were quickly injected into a batch fluidized bed of hot inert particles. The evolution of hydrocarbons was measured by means of an on-line flame ionization detector (FID) located in the product line of the reactor. The FID responses were corrected by taking into account the residence time distribution of the reactor in a convolution with the reaction rate. The initial stages of the reaction are very fast and decreases with increasing particle size. Larger particles also display a time delay for the appearance of product that is related partly to the time required to heat the particle. The later stages of product evolution are slow but continue for long times.