Experimental research was conducted to investigate the reaction of so3 in the presence of water vapor, obtained from a synthetic flue gas mixture, and phosphate rock, to obtain a high conversion of tricalcium phosphate into available P 2 0 5 form, as well as efficient removal of SO3 from flue gas. The investigation was carried out in a fixed-bed reactor, employing Florida pebble rock.Investigations of effects of bed temperature, gas flow, SO3 concentration, and reaction time on so3 pickup and conversion of P2Oj to available form showed that SO3 in flue gases can be removed by and reacted with phosphate rock to produce a product suggestive of fertilizer-grade normal superphosphate. About 87% conversion of total P z O~ present in the starting material to available form was achieved, yielding a product which contained 18.6% by weight of available PzO5. The experimental data were correlated empirically and presented graphically. Application of the data to reactor sizing is illustrated, using a movingbed reactor as an example.