Direct determination of mercury species in real environmental samples is challenging due to their trace concentrations and low ionisation energy. In this investigation, a microwave-assisted and aqua regia digestion method is proposed for the analysis of trace inorganic mercury. Measurements were performed using inductively coupled plasma optical emission spectroscopy (ICP-OES) method in soil, digestate and waste samples. The detection limits of the proposed method were found to be 0.03 and 0.115 mg/kg for mercury at 184.9 and 194.2 nm, respectively. The accuracy of the method with respect to the systematic and constant errors was confirmed by the analysis of certified reference material (CRM) (> 95 % recovery with < 15 % relative standard deviation RSD). The method was successfully applied for the determination of mercury in solid wastes listed in the Decree on Burdening of Soil with Waste Spreading and the Decree on the Treatment of Biodegradable Waste and the Use of Compost or Digestate in the Republic of Slovenia (Official Gazzette of the Republic of Slovenia, No. 34/08 and 61/11 and No 99/13, 56/15 and 56/18).