A mass transfer model using a linear equilibrium isotherm is presented for the static and dynamic stages of supercritical fluid extraction tests. In the static stage the fraction extracted is a function of a dimensionless time and a dimensionless equilibrium parameter. In the dynamic stage the fraction recovered is a function of a different dimensionless time, the same equilibrium parameter, and a dimensionless mass transfer parameter. Fitting the model to experimental extraction curves showed that increasing the pressure or density increases the equilibrium constant and decreases the overall mass transfer coefficient.