The simultaneous absorption of HCl and SO2 by hydrated lime in a fixed-bed reactor has been studied at conditions simulating humidified flue gas dry scrubbing. At 120 degrees C, an increase in relative humidity from 0% to 19% increased HCl removal and sorbent utilization. At 19% relative humidity (RH), the final loading of hydrated lime was 1.64 mol of HCl/mol of Ca2+. From 250 to 1000 ppm HCl, HCl removal was first order in HCl concentration. When SO2 was added to the feed gas, the total utilization of the sorbent by HCl and SO2 was not a function of gas concentration with 250-1000 ppm HCl and 0-2000 ppm SO2. However, the fraction of the hydrated lime converted by SOB increased as the SO2/HCl feed ratio increased. With oxygen present in the feed, more SO2 was absorbed. Adding 150 ppm NO2 to the gas stream increased the final SO2 loading from 0.06 to 0.17 mol of SO2/mol of Ca2+. As the reactivity of SO2 and NO2 increased, the reactivity of HCl decreased slightly as a result of the competition for alkalinity with the other acid gases. The data were modeled using semiempirical flux equations based on a modified shrinking core model. Results from the parameter estimation were used to predict the absorption of HCl and SO2 on the surface of a bag filter. The predictions indicated that, with a humidified flue gas and 50% sorbent utilization, less than 20% HCl penetration is possible. However, even at the conditions with the greatest SO2 absorption (250 ppm HCl, 150 ppm NO2, and 2.5% O-2), 90% SO2 penetration is predicted at 50% sorbent utilization.