Cationic, high-molecular weight polyacrylamide copolymer that viscosifies hydrochloric (HCI) acid for use in carbonate acidizing treatments. Being acrylamide based, AGA-400 works to temperatures of 149°C (300°F). It is the main component of a cross-linked acid system. When combined with AGA-Buffer-1, AGA-XL-1nand ICA-720, a self-diverting, cross-linked acid system is developed.