Isotope ratio measurements characterizing 202Hg/200Hg in NIST SRM 3133 Mercury Standard Solution were undertaken by multicollector inductively coupled plasma mass spectrometry employing NIST SRM 997 Tl for mass bias correction by use of the slope and the intercept obtained from a natural logarithmic plot of each session of measurements of 202Hg/200Hg against 205Tl/203Tl. The calculated value of 1.285333 ± 0.000192 (mean and one standard deviation, n = 40) for the mass bias corrected 202Hg/200Hg was then used for mass bias correction of other Hg isotope pairs. Ratios of 0.015337 ± 0.000011, 1.68770 ± 0.00054, 2.3056 ± 0.0015, 1.3129 ± 0.0013, 2.9634 ± 0.0038, and 0.67937 ± 0.0013 (expanded uncertainty, k = 2) were obtained for 196Hg/198Hg, 199Hg/198Hg, 200Hg/198Hg, 201Hg/198Hg, 202Hg/198Hg, and 204Hg/198Hg, respectively. Reduction of Hg(II) to Hg0 in solutions of SRM 3133 was then undertaken using SnCl2, NaBH4, UV photolysis in the presence of formic acid, and ethylation of Hg(II) using NaBEt4. These reactions induced significant isotope fractionation with maximum values of 1.17 ± 0.07, 1.08 ± 0.09, 1.34 ± 0.07, and 3.59 ± 0.09‰ (one standard deviation, 1SD, n = 5) for δ202/198Hg relative to the initial isotopic composition in the solution following 85–90% reduction of the Hg by SnCl2, NaBH4, UV photolysis, and ethylation with NaBEt4, respectively. Mass-dependent fractionation was found to be dominant for all reduction processes.