A colorimetric sensor is proposed for determination of thiocyanate based on the destruction of mercury–diphenyl–carbazone complex immobilized in the optically transparent polymethacrylate matrix in the concentration range 0.8– 50 mg dm–3 with the detection limit of 0.6 mg dm–3 or visual determination with ‘naked eye’ in the concentration above 1 mg dm–3. The possibility of application of such sensors for determining thiocyanate in fossil and drill waters is shown.