Alloying in quaternary (Mn,Fe)2(P,Si) allows for highly tunable phase transformation temperatures and hystereses, potentially enabling high-efficiency magnetocaloric cooling over a wide range of temperatures. However, compositional control of the alloy is also subject to complex thermodynamic constraints, as evidenced by segregation of multiple transforming hexagonal phases, as well as precipitation of a non-transforming cubic P-poor (Mn,Fe)3Si phase. In either case, the composition, and therefore hysteresis and transformation temperature, of the transforming phases are modified significantly from the nominal bulk composition, thereby obscuring direct causal relationships between composition and transformation behavior. Thus, in order to recover these relationships, as well as to control the expression of transforming phases of interest for cooling applications, it is critical to map out the thermodynamic phase coexistence of the alloy system. In this work, we investigate thermodynamically driven phase segregation behavior in (Mn,Fe)2(P,Si) by measuring the compositions and mass fractions of coexistent transforming and non-transforming phases for a range of nominal alloy compositions (1.20<Mn<1.25; 0.35<P<0.50) through quantitative wavelength dispersive spectroscopy and backscatter electron imaging. It is found that oxygen preferentially segregates to the cubic phase over the hexagonal phase (5 at. % vs. <1 at. %), suggesting oxygen plays some role in mediating the stabilization of the cubic phase. Measured compositions of the expressed transforming hexagonal phases are then combined with critical transformation temperatures and thermal hystereses from calorimetry experiments to map out the underlying dependence of the transformation behavior on the phase compositions. The analysis suggests Mn/Fe site occupancy plays a much larger role than P/Si in controlling hysteresis of the phase transition, providing insight into the nature of the energy barriers that fundamentally control hysteresis in this alloy system.