A candidate reference material of natural isotopic composition for inorganic mercury has been characterized by the Institute for National Measurement Standards of the National Research Council Canada. The material, designated NIMS-1 (natural inorganic mercury standard) is certified for isotope ratios, isotopic abundances and atomic weight of mercury. The certification was achieved using multi-collector ICP-MS based on a state-of-the-art regression model for calibration. The certified isotopic composition is x196 = 0.001 55(4), x198 = 0.100 38(10), x199 = 0.169 38(9), x200 = 0.231 38(6), x201 = 0.131 70(12), x202 = 0.297 43(9) and x204 = 0.068 18(6) with the corresponding atomic weight of mercury Ar(Hg) = 200.5924(8). Values are presented in a concise notation whereby the expanded uncertainty with a coverage factor of two is given in parenthesis next to the least significant digits to which it applies. A full disclosure of all raw data pertinent to certification is presented in order to afford a fully transparent process. Care was taken to ensure high metrological quality in the evaluation of the accuracy and the uncertainty of the certified results. In this regard, it is superior to the best measurement from a single terrestrial source as currently recognized by IUPAC. Considering the known natural variations of Hg isotopic composition, we propose 200.592(3) as a revised assessment of the standard atomic weight of mercury.