This paper investigates the influence of viscous dissipation and radiation on the problem of unsteady magneto-hydrodynamic free-convection flow past an infinite vertical heated plate in an optically thin environment with time-dependent suction. By taking the radiative heat flux in the differential form, and imposing an oscillatory time-dependent perturbation, the coupled nonlinear problem is solved for the temperature and velocity distributions. The effects of the material parameters on the temperature and velocity profiles are discussed quantitatively. The results show that increased cooling (Gr > 0) of the plate and the Eckert number leads to a rise in the velocity profile; while increases in magnetic field, radiation and Darcy parameters are associated with decrease in the velocity. Also, an increase in the Eckert number leads to an increase in the temperature, whereas increases in radiation and magnetic field parameters lead to a decrease in the temperature distribution when the plate is being cooled. (C) 2003 Elsevier Science Ltd. All rights reserved.