This study involves thermochemical decomposition of phosphogypsum (PG) in the presence of oil shale (OS) as the source of carbon in a bench-scale fluidized-bed reactor, with air flowing as the gaseous feed. Experiments were carried out thermochemically under isothermal conditions in a batchwise manner at different added weight fractions of OS varying from 0.1 to 0.3 (w/w). The temperature range considered was in the range of 1123-1273 K. The gas phase obtained from thermochemical decomposition of the sample was monitored by gas analyzers (MSI 5600, Germany; MIR 9000, France),and all process data were recorded on-line at regular intervals by a data acquisition system specifically developed for this purpose. Concentration-time data were evaluated for each run to obtain conversion-time curves. The overall reaction was found to be reaction rate controlled in compliance with the Avrami equation which represented the observed data reasonably well. The rate constant was found to have an activation energy of 184 kJ/mol for the 0.30 weight fraction of OS in the feedstock. The activation energy which was determined using the Avrami equation dropped from 631 kJ/mol at 0.1 weight fraction to 184 kJ/mol at 0.3 weight fraction. Increased operating temperature at a preset feed ratio of OS to PG increased the SO2 concentration and decreased the CO concentration. This indicated the possibility that the complete oxidation of the carbon source would be taking place at higher temperatures as the reducing atmosphere diminished.