Homocysteine (Hcy) and cysteine (Cys) are crucial to the physiological balance in living systems. Specific detection of intracellular Hcy and Cys is of growing importance. Herein, we demonstrated phosphorescence imaging of intracellular Hcy and Cys using a cationic iridium(III) complex Ir(pba)(2)(bpy)(+)center dot PF6- [pba = 4-(2-pyridyl)benzaldehyde, bpy = bipyridine] containing aldehyde groups as a luminescent probe. Upon addition of Hcy or Cys to a semiaqueous solution of Ir(pba)2(bpy)+, a change in luminescence from yellow to red was visible to the naked eye. The successful chemical reaction of the aldehyde in Ir(pba)(2)(bpy)(+) with Hcy and Cys to form thiazinane and thiazolidine was confirmed by H-1 NMR. Moreover, complexation with Hcy and Cys disturbed the p-pi conjugation between the aldehyde group and the bpy moiety, and led to the excited states switching to [d pi(Ir)-pi(star)(N boolean AND N)] (MLCT)-M-3 and [pi(C boolean AND N)-pi(star)(N boolean AND N)] (LLCT)-L-3 from (pi-pi(star))(pba(-)) (IL)-I-3. Furthermore, the MTT assay was used to determine that the probe has low cytotoxicity. Importantly, cell imaging experiments demonstrated that the probe is membrane permeable and can monitor the changes of Hcy/Cys within living cells in a ratiometric mode.