The final step in a sequential extraction procedure for Hg in geological samples usually involves a strong acid attack such as aqua regia. It is assumed that the Hg component measured in this step represents HgS (cinnabar), the common and highly insoluble form of Hg in nature. This paper describes the optimization of an HNO3-based dissolution of all non-sulphide forms of Hg while minimizing the solubilization of HgS. Previous work indicated that 12M (75% v/v) HNO3 would fulfil that objective but its application in this work to samples containing very fine-grained HgS showed that this concentration was too strong as it partially dissolved HgS. Thus, HgS control samples were used to study the effect of HNO3 concentration, duration of extraction and sample-to-volume ratio on the chemical breakdown of HgS. The recommended procedure to separate HgS from other forms of Hg is a two-hour extraction of 1 g of sample in 20 ml of 40% HNO3 with constant agitation followed by a 10-ml rinse with 40% HNO3. This then ensures that all the cinnabar, fine- and coarse-grained, reports to the subsequent aqua regia step.