Laminar free convection flow of air past a semi-infinite vertical plate in the presence of chemical species concentration and thermal radiation effects is studied. This type of problem finds application in many technological and engineering fields such as rocket propulsion systems, spacecraft re-entry aerothermodynamics, cosmical flight aerodynamics, plasma physics, glass production and furnace engineering. The governing boundary-layer equations for this problem are reduced to a non-similar form and are solved numerically by an implicit finite-difference technique. Representative velocity, temperature and concentration profiles are shown graphically and the numerical values of the wall slopes of the velocity, temperature and concentration profiles (which are related to the dimensionless skin-friction coefficient, wall heat transfer and the Sherwood number, respectively) are also shown graphically. The effects of the radiation parameter, buoyancy ratio, Schmidt number and the dimensionless distance from the leading edge of the plate on the numerical solutions are presented and discussed.