A number of chemical reactions (mass-action expressions) have been proposed to describe serpentinization of olivine in terrestrial serpentinites. Some have been applied to interpreting the formation of serpentine-group minerals in CM carbonaceous chondrites. The widely used olivine + pyroxene → serpentine (ol + px → srp) reaction is quantitatively inconsistent with observed ratios of olivine to pyroxene and olivine to serpentine in CM carbonaceous chondrites. The reaction of 5ol → 2srp is most consistent with constraints from textural observations at the scale of individual partial and complete pseudomorphs of serpentine after coarse olivine in CM chondrites, observed co-variations in modal abundances of reactant and product minerals in CM chondrites, and experimental geochemical kinetics. Furthermore, the 5ol → 2srp reaction provides insight into the mobility of solute species in the aqueous alteration environment of CM carbonaceous chondrites.