A method and apparatus for removing acid gases such as halogen acids and sulfur oxides from a hot flue gas by contacting the gas with a finely-divided, pre-moistened and partially-agglomerated, intimate mixture of a solid alkaline reagent and a deliquescent compound. The dry akaline reactang/deliquescent compound mix is pre-conditioned by exposure to a gas of controlled humidity for a time sufficient to cause partial liquefaction of the deliquescent compound. Contact at controlled humidity conditions may be effected in a treating vessel or in a transport duct prior to injection into the hot flue gas. Alternatively, the dispersed deliquescent compound may be separately liquefied prior to mixing with the alkaline solids. Liquefaction of the deliquescent compound is achieved by controlling the partial pressure of water vapor in the conditioning gas mixture at a level higher than the equilibrium vapor pressure of the liquefied deliquescent material or its highest hydrate. In these moistening methods, the ratio of deliquescent to alkaline solids feed rates is so adjusted as to avoid the buildup of wetted material on the duct walls.