Purpose – The purpose of this paper is to deal with unsteady double diffusive natural convection in a square enclosure filled with a porous medium with various boundary condition effects in the presence of heat source or sink.

Design/methodology/approach – Finite difference method was employed to solve the dimensionless governing equations of the problem. The effects of governing parameters, namely, amplitude wave length ratio, dimensionless time parameter, Darcy number, buoyancy parameter and heat generation/absorption parameter on the streamlines, temperature and concentration contours, as well as Nusselt number and Sherwood number, were considered.

Findings – The sinusoidal variations of the temperature and concentration remove the singularities which appear in the case of fixed temperature and concentration.

Originality/value – The paper's results are validated by favorable comparisons with previously published results. The results of the problem are presented in graphical and tabular forms and discussed.