The charring of samples and loss of mercury by a nitric-sulfuric acid mixture has been overcome by prior digestion with nitric acid, followed by the addition of sulfuric acid into the final solution. This approach provides optimum decomposition and analytical conditions for a reliable cold vapor atomic absorption spectrometric determination of mercury in biological and environmental materials. In all cases, the presence of sulfuric acid improved the sensitivity of the mercury response significantly. Furthermore, the prior- and post-addition of sulfuric acid gave similar sensitivity and mercury concentrations for all samples. The adequacy of the prior digestion with nitric acid and post-addition sulfuric acid for reliable mercury determination was successfully demonstrated for blood, bone, bovine liver, mushroom, lake sediment, peat and prawn samples.