A new homocysteine-selective sensor based on the iridium(III) complex Ir(pba)(2)(acac) (Hpba = 4-(2-pyridyl)benzaldehyde; acac = acetylacetone) was synthesized, and its' photophysical properties were studied. Upon the addition of homocysteine (Hcy) to a semi-aqueous solution of Ir(pba)(2)(acac), a color change from orange to yellow and a luminescent variation from deep red to green were evident to the naked eye. The blue-shift of the absorption spectrum and enhancement of the phosphorescence emission upon the addition of Hcy can be attributed to the formation of a thiazinane group by selective reaction of the aldehyde group of Ir(pba)(2)(acac) with Hcy, which was confirmed by H-1 NMR studies. Importantly, lr(pba)(2)(acac) shows uniquely luminescent recognition of Hcy over other amino acids (including cysteine) and thiol-related peptides (reduced glutathione), in agreement with the higher luminescent quantum yield of the adduct of Ir(pba)(2)(acac) with Hcy (0.038) compared with that of the adduct with Cys (-0.002). Both surface charge analysis and the electrochemical measurement indicated that a photoincluced electron-transfer process for lr(pba)(2)(acac)-Cys might be responsible for the high specificity of lr(pba)(2)(acac) toward Hcy over Cys.