The Magma Chamber Simulator (MCS) is an open system thermodynamic model for computing the evolution of a magma body as it thermally and reactively interacts with surrounding country rock via assimilation (A) of wallrock partial melts and/or stoping (S) of wallrock and/or undergoes magma mixing by recharge (R) during cooling and concomitant fractional crystallization (FC). The composite system consists of multiple sub-systems: resident Magma, Wallrock, and multiple (0 to 30) Recharge magma reservoirs or stoped block compositions. The MCS uses one of four MELTS phase equilibria engines coupled to a user-defined executive (IGOR) to track and report phase equilibria and major element, trace element and isotopic concentrations within all sub-systems based on equilibrium thermodynamics. The flexible user-interface affords many options that can be tailored to model specific natural systems or model analogs. For example, in addition to full RnASmFC (0 ≤ n+m ≤ 30) scenarios, assimilation-fractional crystallization (AFC), Stoping –Fractional Crystallization (SmFC), recharge-fractional crystallization (RnFC) or fractional crystallization only (FC) simulations may be modeled. A user-generated input MES file (MES_name) is used to define the initial conditions of particular petrological scenarios. MCS supports up to 48 trace elements including the radiogenic elements (Sr, Nd, 206Pb, 207Pb, 208Pb, Hf and Os) and the stable isotope oxygen. The MCS runs on both PC’s and Mac’s. Details and downloads are given on the MCS Code tab.