A selective matrix separation/enrichment method, utilizing a simple batch procedure with nickel/nickel boride (Ni/NixB) nanoparticles was proposed for the determination of inorganic mercury(II), Hg2+ and methyl mercury(I), CH3Hg+ in waters prior to cold vapor-atomic fluorescence spectrometry (CV-AFS). The Ni/NixB nanoparticles, were synthesized by the chemical reduction of Ni(II) to Ni/NixB. The novel adsorbent was selective to Hg2+ and CH3Hg+ species between pH values of 4 - 10. Both of the mercury species were recovered from the adsorbent using 1.0 mol L-1 hot HNO3 with high efficiency. It was observed that the adsorbent selectively removed Hg2+ and CH3Hg+ from the bulk solution in the presence of several competitor ions (As3+, Sb3+, Pb2+, Zn2+, Cu2+, Cd2+ and Fe3+) with >= 96% adsorption. The limit of detection (3 sigma above blank) was found to be 1.8 ng L-1 with a preconcentration factor of 20. The validation of the method was tested through spike recovery experiments with several water samples (tap and seawater) at mu g L-1 concentration levels, and all recovery values were found to vary between 95 and 105%.