The noncoordinated hydroxyl group of [Co(dhpta)](-) (where dhpta = 1,3-diamino-2-hydroxypropane-N,N,N’,N’-tetraacetate ion) was O-acylated by reaction with a series of acid anhydrides (acetic anhydride, n-valeric anhydride, benzoic anhydride, phthalic anhydride, succinic anhydride, octanoic anhydride, and dodecanoic anhydride) in the presence of 4-(dimethylamino)pyridine, after the potassium salt of the complex was dissolved in acetonitrile by using cryptand 222. Fairly high yields under remarkably mild conditions were achieved without altering the coordination sphere. The esterified complexes were characterized by elemental analysis, electronic absorption and NMR spectroscopies, and X-ray crystallography. X-ray crystal structure determinations of (DMAP-H)[Co-(Ben-dhpta)] (10) and (STC-H)[Co(Val-dhpta)].6H(2)O (11) are reported, where DMAP-H, Ben-dhpta, STC-H, and Val-dhpta represent 4-(dimethylamino)pyridinium cation, O-benzoylated dhpta, strychnine cation, and O-valerylated dhpta, respectively. Crystal data for 10 : formula C25H29N4O10Co, space group P2(1)2(1)2(1) (orthorhombic); a = 12.5709 (9) Angstrom, b = 25.338 (3) Angstrom, c = 8.0707 (6) Angstrom, Z = 4, R = 0.1034. Crystal data for 11 : formula C37H57N4O18Co, space group P2(1) (monoclinic); a = 19.498 (10) Angstrom, b = 11.625 (5) Angstrom, c = 9.038 (2) Angstrom, beta = 95.33 (3)degrees, Z = 2, R = 0.072. In both complexes, the cobalt is octahedrally coordinated with a hexadentate dhpta unit. The oxygen atom of the noncoordinated hydroxyl group of dhpta binds to the carboxylic carbon atom of each acid residue to form an ester bond. Measurements of cobalt-59 NMR for the aqueous solutions of the cobalt(III) complexes having a long alkyl (octyl and dodecanoyl) chain ligand indicated the formation of ion-aggregates above cmc (critical micelle concentration), which was determined from the surface tension.