Some theoretical aspects of the onset of buoyancy-driven instability in an initially quiescent, isotropic fluidsaturated porous layer are considered. Darcy’s law is employed to examine characteristics of fluid motion under the Boussinesq approximation. Using linear theory, we derive stability equations and transform them in the similarity domain. Based on linear stability equations in the similarity domain, we prove the principle of exchange of stabilities and show that the stability parameter is stationary. The temperature disturbance field is expressed as a series of orthonormal functions and the vertical velocity one is obtained in simple recursive form. The validity of the quasi-steady state approximation (QSSA) is also proved by comparing the stability characteristics under the QSSA with those obtained from the eigenanalysis without the QSSA.