That some value of the f (sub S 2 ) /f (sub O 2 ) ratio must be exceeded to stabilize sulfide immiscibility in gabbroic magmas is the working hypothesis for the formation of platinum-enriched horizons in the Bushveld and Stillwater Complexes. Intrinsic f (sub O 2 ) measurements from 900 degrees to 1,160 degrees C are given for eight eastern and three western Bushveld samples representative of the chromitites within 970 m of the stratigraphy containing the reef. Similarly, f (sub O 2 ) -T data for five samples of anorthosites and one sample of chromitite are given for 370 m of the stratigraphy enclosing the Stillwater J-M reef platiniferous horizon.In the eastern Bushveld, the f (sub O 2 ) decreases (more reduced) with elevation; in the Merensky Reef footwall about a three order of magnitude drop from f (sub O 2 ) = 10 (super -11) to f (sub O 2 ) = 10 (super -13.7) was measured at 1,150 degrees C for the respective horizons of -869.8 m and -687 m (base of the Merensky Reef as zero datum elevation). From the -687-m horizon to the Merensky Reef, the f (sub O 2 ) values at 1,150 degrees C range between 10 (super -13) to 10 (super -14) with one western Bushveld Merensky Reef sample at 10 (super -15.6) .In the Stillwater Complex, original igneous f (sub O 2 ) values and later serpentinization produce f (sub O 2 ) values at 1,150 degrees C that vary from 10 (super -12.0) (deep footwall) to 10 (super -9.8) (zone of interest). The serpentinization has erased the original igneous redox equilibria for several meters on each side of the zone of interest.Six lines of evidence are reviewed that suggest the Bushveld Merensky event was the result of a system open to volatile composition changes and that the Bushveld chamber may have hovered close to the sulfide immiscibility during 677.3 m of accumulation. In our model, at the Merensky formation time a final perturbation in the f (sub S 2 ) /f (sub O 2 ) ratio triggered the sulfide immiscibility. Spatial constraints and mass balance calculations are presented to test this model.The Merensky Reef and the Stillwater zone of interest both display visual graphite, potholes, and enigmatic platinum-group element concentrations, as well as other discussed similarities that suggest the presented model for the Bushveld may have been originally operative for the Stillwater as well.