The smoothing of weakly roughened GaAs (10 0) substrates is measured with elastic light scattering during homoepitaxial growth of GaAs buffer layers. The smoothing measurements are used to determine the coefficient of the linear term in the continuum growth equation for GaAs, as a function of growth rate and temperature. The temperature and growth rate dependence are in good agreement with theoretical predictions from an atomistic description of the growth process. The density of atomic steps on the surface, which is a key parameter in the continuum growth equation, is measured independently using atomic force microscopy. The linear smoothing coefficients computed from the experimental values for the step density, are found to be in good agreement with the smoothing rates measured with light scattering. These experiments provide experimental support for the continuum growth model that has been derived analytically from basic atomic-level phenomena in epitaxial film growth. (C) 2008 Elsevier B.V. All rights reserved.