Photocatalysis has been established as an emerging technology for wastewater treatment since photocatalytic activity of number of semiconducting materials has already been established. For a number of chemical reactions, ternary oxides have been used as photocatalysts. In the present investigation, ternary oxide of lanthanum has been synthesized using co-precipitation. Synthesized catalyst has been characterised using X-ray diffraction patterns. Photocatalytic bleaching of methylene blue using lanthanum's ternary oxide was observed spectrophotometrically. The effect of variation of different parameters like concentration of methylene blue, pH, amount of semiconductor and light intensity was observed on the rate of photocatalytic bleaching. A tentative mechanism for the photocatalytic bleaching of methylene blue has been proposed.