The transition from steady to oscillatory flow for a very low Prandtl number fluid (Pr = 0.008) is computed for rectangular enclosures with aspect ratios (length/height) of 0.25, 0.4, 1.0, and 2.0 and are found to occur at Rayleigh numbers of 250,000, 130,000, 83,500, and 30,000 respectively. The structures of the oscillations are graphically depicted and are manifested in corner cells which dissipate into centered cells and then into opposite corner cells. A secondary flow transition is detected for a geometry with an aspect ratio of 1.0 at Ra = 1.2Ra(c2). (c) 2004 Published by Elsevier Ltd.