A new type of ratiometric luminescence-based pH sensor is described. The spatial segregation of vanadium ions of different oxidation states within the Y₃Al₅O₁₂ (YAG) nanoparticles induces a difference in the susceptibility of their spectroscopic properties to a change in the pH of the medium. Hence, the emission intensity of V⁵⁺ localized mainly in the surface part of nanocrystals is more efficiently quenched by vibrational modes of hydroxyl groups compared to that of V³⁺ localized mainly in the inner part of the nanocrystal. Thus, their emission intensity ratios were found to be monotonically pH dependent in the 2–11 range, with a high relative sensitivity of S_Rmax of 6%/pH at 7 pH. To the best of our knowledge, this is the first ratiometric luminescence pH sensor based on transition metal ions.