Two-dimensional steady free convection from an isothermal vertical plate is studied in a gas where a reversible very fast reaction of dissociation A<->2B takes place at atmospheric pressure. The effective properties in the presence of dissociation are evaluated. The governing boundary-layer equations are solved numerically for a wide range of values of the independent variables. All the data obtained are correlated by a single correlation even if the temperature interval in the boundary layer (T-W, T-infinity) is allowed to vary in a wide range, both in relative location and width, in respect to the temperature interval of dissociation. The correlated dimensionless parameters include the ratio rho(W)/rho(infinity) and are defined through the mixture effective properties calculated at T-W and T-infinity. The maximum absolute value of the relative error results to be dependent essentially by two particular parameters related to the variations of alpha and rho* in the boundary layer. (C) 2000 Elsevier Science Ltd. All rights reserved.