The complex equilibria between [M(NH3)5(OH2)]3+ (M = Co, Cr) and HCrO4- have been determined spectrophotometrically in acidic aqueous solution (μ = 0.50 M). The stability constant, Kc = [M(NH3)5(HCrO4) 2+]/[M(NH3)5(OH2)3+][HCrO4-, is 1.55 ± 0.02 M-1 for M = Co and 3.56 ± 0.62 M-1 for M = Cr at 25°C. The chromato complex exists in both [M(NH3)5(HCrO4)]2+ and [M(NH3)5(CrO4)]+ forms. The acidity constants for the protonated chromato species at 25°C are (4.43 ± 0.05) x 10-3 M (Co) and (3.37 ± 0.63) x 10-2 M (Cr). The temperature dependence of the chromium(III) complexation with HCrO4- is described and the resultant thermodynamic parameters are discussed in relation to other chromate reactions.