A simple and new method that can be used to determine the mechanism by which nonplanar surfaces relax to flatness is described. Corrugated surfaces with a non-sinusoidal profile are irradiated with laser light. The intensity of the light diffracted in the various orders is measured during isothermal heating of the sample. The relative decrease of the light intensity in the different diffraction orders identifies the flattening mechanism and the measurement of the rate constant allows the calculation of the pertinent material parameters. This method is illustrated by measurements performed on amorphous Se films. It is shown that the dominant flattening process is viscous flow. Upon annealing, the corrugated surface assumes a sinusoidal shape of a fixed wavelength but whose amplitude decays exponentially with time. A value of 2.12 × 10 -10 m s -1 is deduced for the ratio of the surface tension to the viscosity from the time decay rate at a temperature of 40°C, below the glass transition temperature of 45°C.