Locally sourced periwinkle shell ash (PSA) and snail shell ash (SSA) were investigated as potential photocatalysts for the photocatalytic decolourisation of methylene blue in aqueous solution. Preliminary studies showed that PSA performed better than SSA hence it was adopted for further investigations. The effects of process variables such as irradiation time, initial dye concentration and catalyst dosage on the decolourisation process were evaluated. The optimum values of the process variables were: irradiation time, 50 minutes; initial dye concentration, 7 mg/L and PSA dosage, 2 g/L. The adsorption equilibrium was well described by the Langmuir isotherm equation (R²=0.999) indicating mono layer type adsorption while the kinetics of the process were well described by the pseudo first order and Langmuir-Hinshelwood kinetic models with high R² values (>0.90).