Arsenazo I was used as a metallochromic indicator for the spectrophotometric determination at 560 nm to 570 nm of apparent stability constants of magnesium and calcium complexes of tricarboxylates and ADP between pH 7.4 and pH 8.0. Average values of apparent stability constants (nM−1) in 0.14 M Tris-HCL at pH 8.0 obtained with this method are reported sequentially for the magnesium and calcium complex in the parenthesis after each compound: arsenazo I (1.62, 0.805), ADP (2.01, 0.606), citrate (2.10, 2.57), O-methyl citrate (3.84, 1.94),DL-erythro-fluorocitrate (0.538, 0.736),DL-threo-isocitrate (0.432, 0.435),DL-threo-α-methylisocitrate (0.496, 0.240),DL-erythro-α-methylisocitrate (0.161, 0.287),DL-threo-homoisocitrate (0.127, 0.192), tricarballylate (0.238, 0.143), 3-hydroxyglutarate (0.020, 0.016), and the hydroxycitrate diastereoisomers garciniate (0.368, 0.988) and hibiscusate (0.771, 1.21). The constant for magnesiumDL-methylcitrate was 3.51 mM−1.