The present work is devoted to the impacts of thermal radiation on the unsteady laminar convective MHD flow of a viscous electrically conducting fluid having a temperature-dependent viscosity over a rotating porous disk of infinite extend impulsively set into motion. The fluid is subjected to an external uniform magnetic field perpendicular to the plane of the disk. The governing Navier-Stokes and Maxwell equations of the hydromagnetic fluid, together with the energy equation, are reduced into a system of nonlinear ordinary differential equations via the Von Karman similarity transformations. Due to the radiation effect and variable viscosity property the equations are highly coupled. These equations are then solved numerically by using a technique based on the spectral Chebyshev collocation in the direction normal to the disk and forward marching in time. Transient effects are discussed first. Later a parametric study of all parameters involved is performed and a representative set of results incorporating the effects of Prandtl number, the viscosity variation parameter, the magnetic interaction parameter, the radiation parameter, the uniform suction/blowing parameter, the relative temperature difference parameter and the Eckert number on physically meaningful quantities such as the radial and tangential skin friction coefficients, the torque, the vertical wall suction and the rate of heat transfer from the disk surface are illustrated graphically and discussed. (C) 2010 Elsevier Masson SAS. All rights reserved.