A numerical study is presented of the effects of valve on mixing characteristics of two fluids inside a closed two-dimensional cavity. A two-fluid model is employed which involves solution of separate transport equations for zone-averaged variables of each fluid with allowance for interface friction. The calculations are performed over the parameter range : Grashof number (1-3.6 x 10(5)), Atwood number (5 x 10(-5)-0.33) and valve speed (0.025-0.1 m/s). The mixing efficiency is presented as a function of valve speed and the dynamical system characteristics of the flow field are discussed in terms of the time histories, phase space trajectories and power spectra of the velocities at a fixed point. The effects of valve and buoyancy forces are found to be comparable at high Grashof numbers and moderate valve speeds. However, the perturbation induced by the valve dominates the mixing at low Grashof numbers.