The transport rates of citric acid (HA) in supported liquid membrane module containing a carrier of tri-n-octylamine (TOA) were measured using a batch type, and a simple model (HA + TOA <-> complex) was applied to investigate the mass transfer mechanism of HA with TOA. The measured values of the forward- and backward-reaction rate constant in the reversible reaction between HA and TOA were used to simulate the model with the mass conservation equation and associated boundary conditions. Comparison between the experimental and simulated facilitated factor of HA transport led to classification of reaction regions.