  Effect of interfacial disturbances on instabilities of
buoyant/thermocapillary convective flows in rectangular cavities is studied in
a series of numerical experiments. The computations are carried out for several
two-liquid two-layer systems taking into account properties of liquids used in
previously published experiments. Relation between the interface deformations
and the Boussinesq approximation is discussed. It is shown that in some
systems, including the interface disturbances in the model can alter the
critical temperature difference by approximately 10%, producing either
destabilizing, or stabilizing effect. The interface oscillations appear as
standing or travelling waves whose wavelength can vary from short wave lengths
to a single wave occupying all the available space. Rough estimations show that
in some liquid-liquid systems the interface oscillations amplitude can reach
several tens of microns. Patterns of the most unstable disturbances are
presented and discussed. It is argued that instabilities in some two layer
systems develop similarly to the Holmboe instabilities in stratified mixing
layers.
