Linear stability analysis has been carried out to predict the onset of double-diffusive convection in superposed fluid and porous layers using a one-equation model. The eigenvalue problem is solved numerically by a finite difference scheme. Results have been obtained for the thermal convection and salt-finger cases. Comparing with the results obtained for the same problems by Chen and Chen [F. Chen, C.F. Chen, J. Heat Transfer 110 (1988) 403-409] using a two-equation model, we find that these two methods give the same general characteristics of the marginal stability curves, however, there are differences in the critical conditions and the flow streamlines at onset. Carefully conducted experiments are needed to determine which model gives the more realistic results.