The ability of Triton X-100 to affect thermooptical parameters of water is used for determining the surfactant by thermal lensing. The limit of detection of Triton X-100 in the presence of pre-synthesized cobalt(III) tris-(2-nitroso-1-naphtholate) as an inert colorant is 8×10-4 mol dm-3, which is close to the critical micelle concentration of the surfactant. A method for estimating the limits of detection of thermal lensing in a given solvent system based on the reproducibility of measurements for a single sample is developed. In this approach, Triton X-100 is applied for enhancing the sensitivity of thermal lensing in aqueous solutions. For the concentrations of Triton X-100 of 0.04 - 0.1 mol dm-3, a twofold enhancement in detection limits of iron(II) with 1,10-phenanthroline and cobalt(III) with 2-nitroso-1-naphthol as model systems without any change in the sample preparation is observed.