γ‐Carboxyglutamic acid (Gla) is believed to bind Ca [II] ions and Mg [II]ions in prothrombin and other coagulation proteins. Binding constants for H+, Ca [II] ions, and Mg [II]ions to Gla‐containing peptides are determined using pH and ion selective electrode titrations. The binding constants for peptides containing a single Gla residue are similar to the constants for malonic acid. Peptides containing two Gla residues in sequence (di‐Gla peptides) bind Ca [II] ions and Mg [II]ions more strongly. KMgLfor the di‐Gla peptides is similar to the site‐binding constant for Ca [II] ions in denatured BF1. These di‐Gla peptides may be useful analogs for metal binding by the disordered Gla d