A solar pond is a basin of water where solar energy is trapped due to an artificially created gradient of salinity that prevents convective motions. The present study intends to clarify the contribution of non-constant diffusion coefficients for the stability of the gradient layer together with the influence of solar radiation absorption, the thermal and molecular diffusivities bring assumed to be linear functions of the vertical co-ordinate z. The analysis shows that the consideration of these two effects decreases the margin of stability in comparison with previous studies based on a layer of fluid heated from below with constant diffusivities coefficients and linear profiles for both temperature and salt.