Four of the most commonly used wet digestion methods were evaluated for the accurate determination of mercury in biological and environmental materials by cold vapour atomic absorption spectroscopy. The method based on the ue of HNO3-H2SO4 mixture was found to be most suitable for the reliable determination of the element. The recovery efficiencies of the method for mercury in fish homogenate and horse kidney samples were 95.9 and 105.7%, respectively for the inorganic form, and 89.0 and 99.8% for the organic form. The method based on the use of HNO3-H2O2 mixture also gave similar recoveries for the organic and inorganic mercury in these samples. In comparison, the recoveries of the organic mercury in the fish and kidney samples by the use of HNO3 only and HNO3-HClO4 mixture were < 50%, but their recovery efficiencies for the inorganic mercury were close to 100%. The application of the method utilizing the HNO3-H2SO4 mixture to a range of biological and environmental standard reference materials gave the most accurate and reproducible analytical data. The suitability of the digestion method for the reliable determination of mercury in soil samples was also demonstrated.