A new model is proposed to interpret metastable zone widths (MSZWS) obtained for anti-solvent crystallization. The MSZW is defined as an excessive anti-solvent composition or supersaturation at which the number density of accumulated grown primary nuclei has reached a fixed value when the anti-solvent is continuously added at a constant rate into the initial saturated solution. In the model, the steady-state primary nucleation in the bulk solution is assumed to be achieved instantaneously at any time. The effects of stirrer speed, anti-solvent addition rate, MSZW detection method and anti-solvent addition location are interpreted for the literature data measured for the anti-solvent crystallization of benzoic acid [D. O'Grady, M. Barrett, E. Casey, B. Glennon, Trans. Chem. Eng. Res. Des. 85 (A) (2007) 945]. (C) 2008 Elsevier B.V. All rights reserved.