The authors study linear stability of natural convection flow in an arbitrarily inclined rectangular enclosure along two opposite walls with prescribed heat or mass fluxes. The problem is solved by a finite difference technique with automatically calculated differencing molecules, with variable order of accuracy, and with accurate boundary treatment. It is observed that the case of uniform fluxes is more stable than the fix-wall temperature case. A wide range of Prandtl numbers is considered, and it is observed that instabilities tend to be favoured by a decrease in Prandtl number, an increase in Rayleigh number, and a decrease in the inclination of the box. The results of the paper can be extended to MHD flows.