Light‐induced changes in the refractive index of transition metal doped SrTiO3has been observed. This is the first observation of photochromic index of refraction changes. The measurements are made by observing the variation of transmission of a Fabry‐Perot cavity made of the material studied. Changes in the index of refraction of 1–5 × 10−5per unit change in optical density per centimeter are found. These values agree reasonably well with values calculated from the measured absorption curves.