An analysis is made to investigate the mass transfer in two-dimensional boundary layer flow past a flat plate moving in parallel or reversely to a free stream with first order chemical reaction. The similarity transformations are used to transform the governing equations and the reduced nonlinear self-similar ordinary differential equations are solved numerically using a shooting method. The dual solutions of the velocity and concentration distributions are obtained. With increase of the Schmidt number the mass transfer is found to be enhanced for the upper solution branch and reduced for the lower solution branch. The concentration overshoot for constructive chemical reaction is observed, i.e., for constructive reaction mass absorption occurs. Moreover, for destructive chemical reaction the mass transfer increases. (C) 2012 Elsevier Ltd. All rights reserved.