A pulsed, two-beam, thermal lensing experiment was performed to determine the concentration of aqueous solutes above the critical point of water. Despite a very significant mirage effect due to thermal gradients in the cell and absorption by water itself, the thermal lensing signal strength for aqueous benzoic acid in supercritical water was found to be linear with concentration in the sub-millimolar range. Although thermal lensing experiments in aqueous media are notoriously insensitive, the sharp density gradient near the critical point considerably improves the signal intensity. In this study a short-pulse pump 266 nm YAG laser and continuous low-power probe Ar ion beam were both focused into a supercritical water cell, giving a lensing signal whose strength could be maximized by changing the overlap of the two beams.