A series of carbonate-containing antacids was examined by IR and X-ray analysis to establish the role of carbonate and to compare the structure of the antacids to naturally occurring carbonate minerals. Based on IR analysis, the relative degree of perturbation of carbonate increases in the order calcium carbonate, carbonate-containing aluminum hydroxide gel, and dihydroxyaluminum sodium carbonate. The crystalline carbonate-containing antacids were poorly organized forms of the minerals calcite, CaCO3; dawsonite, NaAl(OH)2CO3; and hydrotalcite, Mg6Al2CO3(OH)16-4H2O. Amorphous carbonate-containing aluminum hydroxide gel can be classified mineralogically as amorphous aluminum hydroxycarbonate. IR and X-ray evidence indicates that magaldrate has a hydrotalcite-like structure with sulfate as the major interlayer anion and carbonate also present in the interlayer space.