The laboratory testing of the reactions of acetic, formic, hydrochloric, and hydrofluoric acids with calcium carbonate, kaolin, sepiolite, and two formation materials at geothermal temperatures is described. A workable test procedure was developed which provided information regarding the relative reactivities of selected minerals or formation materials with three of the four acids investigated. Tests with hydrochloric acid were complicated by reactions of the acid with the test vessel materials and therefore, only very limited work could be done with this acid at the desired temperatures. In spite of these difficulties, information regarding the amount of soluble material in the various acids was obtained. From this information an approximate value for the percent dissolution of the minerals under the different reaction conditions could be calculated. Additional information regarding the formation of solid secondary reaction products upon cooling of the reacted acid was also obtained. The implication of the mineral reactivities with the different acids and the formation of secondary solids on geothermal acidizing operations are discussed. Some selected scale inhibitors (for calcium carbonate) were tested for their hydrothermal stability. Their efficiency in inhibiting the formation of calcium carbonate scale before and after aging at 500/sup 0/F was measured. The implications of the loss of efficiency of these materials and recommendations for their use in the field are discussed.