Accurate molecular structures of iron(III) and gallium(III) complexes of structurally related model hydroxamic acids were investigated by single-crystal X-ray diffraction at –135(2)°C: [Fe(mpa)₃]·Me₂CO 1[mpa =N-(4-methylphenyl)acetohydroxamate], monoclinic, space group P2₁/a, a= 10.635(3), b= 12.137(2), c= 23.667(6)Å, β= 102.34(2)°, Z= 4, R= 0.030 for 5115 reflections; [Fe(mmb)₃]·H₂O·Me₂CO 2(mmb =N-methyl-4-methylbenzohydroxamate), monoclinic, space group P2₁/a, a= 10.852(2), b= 30.690(6), c= 10.494(2)Å, β= 117.89(2)°, Z= 4, R= 0.034 for 6381 reflections; [Ga(mmb)₃]·H₂O·Me₂CO 3, monoclinic, space group P2₁/a, a= 10.822(4), b= 30.718(14), c= 10.463(5)Å, β= 118.51(3)°, Z= 4, R= 0.036 for 6333 reflections. All three complexes crystallize in their enantiomeric Λ- and Δcis configurations. Structural differences between 1 and 2 were used to develop a set of structural criteria to establish the relative stability of the two compounds. These criteria were than applied to compounds 2 and 3 leading to the conclusion that the gallium(III) complex is thermodynamically more stable than the iron(III) complex. It is shown, however, that when the large differences in the free energy of formation for the aqueous ions are taken into account these conclusions are in agreement with the thermodynamic observations that the formation constant of the iron(III) hydroxamate complex is larger than that of its isomorphous gallium(III) complex, and that Fe³⁺ displaces Ga³⁺ in hydroxamato complexes of the latter.