An opto-electrochemical phosphate-ion sensor was developed with a cobalt oxide thin-film electrode. The cobalt oxide (Co3O4) thin-film electrode showed a remarkable change of transmittance at 550-800 nm, under application of an anodic potential at 0.4 V vs. SCE, which was dependent on HPO42- concentration. The change of transmittance at 620 nm, the sensor signal, was almost linear to the logarithm of the HPO42- concentration between 10(-6) and 10(-2) M. The 90% response time, when the electrode potential was changed from +0.4 to 0 V vs. SCE at 10(-2) M, was about 50 s at room temperature. The Co3O4-based optical sensor hardly responded to NO3- or Cl- at the same sensing condition.