The effects of CO2 on water-basaltic glass interaction have been simulated at 25°C. The calculations indicate that addition of CO2 (2–30 bar) to water significantly changes the reaction path. Initially, the pH is buffered between 4 and 6 by CO2 ionization, with dissolution of basaltic glass and the formation of secondary minerals with SiO2, Mg-Fe carbonates and dolomite predominating. Upon the dissolution of additional basaltic glass and mineral fixation of CO2, the pH increases to >8 and (Ca)-Fe-Mg smectites, SiO2, Ca-Na zeolites and calcite become the dominant secondary minerals forming. The overall reaction path depends on the initial water composition, reactive surface area, and the composition of the phyllosilicates and carbonates forming. The key factors are the mobility of Mg2+, Fe2+ and Ca2+ and the competing reactions for these solutes among secondary minerals.