The steady state and the stability behaviour of a double-diffusive natural circulation loop are investigated theoretically. The fluid in the thermosyphon consisting of two vertical channels is subjected to both temperature and salinity gradients. The governing equations of continuity, momentum, energy and mass diffusion are written based on a one-dimensional model for laminar flow. The steady-state solutions include the trivial ‘conductive’ one of no flow in the loop, v = 0, and ‘convective’ flow solutions when the thermal Rayleigh number, R T , exceeds a critical value, R Tc , which depends on the saline Rayleigh number, R s , and the inverse Lewis number, q. In a certain range of system parameters double ‘convective solutions’ are obtained. The results of linear stability analysis show that the one with lower v is always unstable and all the other steady flow solutions are stable. The stability chart in the plane R s - R T , derived by this analysis, also includes a description of instabilities associated with the onset of motion from rest. The marginal stability boundaries include the monotonie marginal stability lines (MMSL) R T = 6 + qR s , and oscillatory ones (OMSL) which depend also on the Prandtl number and bifurcate from the MMSL. An interesting phenomenon, exhibited from the solution, is that the OMSL intersects the line R Tc , representing the critical conditions for existence of steady flow solutions.