Calcium molybdates (undoped and carbon-doped) have been used as a photocatalyst for the degradation of malachite green. These have been characterized by different techniques such as FE-SEM, EDX and XRD. The progress of the reaction has been monitored spectrophotometrically. Different rate affecting parameters like pH, concentration of dye, amount of semiconductor and light intensity have been studied and their optimum values obtained are pH 9.7, concentration of malachite green as 5.00 × 10^-4 M, 0.12 g of CaMoO₄ and light intensity as 50.0 mWcm^–2. A tentative mechanism for the photocatalytic degradation of dye has been proposed, where hydroxyl radical has been observed as an active oxidizing species.