MacArgon allows for the simulation of argon retention in minerals and rocks. Arbitrary pressure-temperature-time paths can be input that a rock must follow, and calculations are performed as to the diffusion of argon from the mineral grains in question. The program allows fact distributions of diffusion domain size and volume that allow replication of data obtained from temperature-controlled furnace-step-heating experiments in an ultra-high-vacuum mass spectrometer designed to measure the concentration of the isotopes of argon. By considering bulk fusion the program also replicates the answers likely to be obtained using laser-spot analysis, or laser-step-heating by pulse-heating the crystals. The program also allows parametric inversion, by considering the variation obtained in families of pressure-temperature-time paths. Manual inversion takes advantage of the highly interactive nature of the graphical user interface.