We demonstrate the Soret effect in a lyotropic liquid crystal at different temperatures across the isotropic phase detected by the time-resolved thermal lens technique. We investigate thermo-optical and mass diffusion properties of the sample and present quantitative characterization of Soret, mass and thermal diffusion coefficients. The results reveal that the temperature gradient induced in the experiments causes the migration of micelles from hotter to colder regions in the sample. The increase of the mass diffusion coefficient with temperature is related to the decrease of the radius of the micelles in the solution. •Soret effect in a lyotropic liquid crystal at different temperatures across the isotropic phase;•Investigation of thermo-optical and mass diffusion properties;•Quantitative characterization of Soret, mass and thermal diffusion coefficients;•The temperature gradient in the experiments causes the migration of micelles from hotter to colder regions in the sample;•The increase of the mass diffusion coefficient with temperature is related to the decrease of the radius of the micelles.