A temperature gradient in a liquid mixture causes a concentration gradient through the Soret effect. We have developed an instrument to measure the Soret effect by observing the bending of a laser beam propagating horizontally through the liquid mixture subjected to a temperature gradient in the vertical direction. Our design of the liquid cell, with a long path length and controlled temperature uniformity, enables us to measure Soret coefficients with an accuracy of 1-3%, higher than that obtained by previous investigators. In addition, by measuring the dynamic response of the beam deflection after imposition of the temperature gradient, we can also determine the mutual diffusion coefficient. We have applied the technique to mixtures of toluene and n-hexane over the temperature range 5-45 degrees C and to mixtures of ethanol and water at 25 degrees C. We have verified that the measured value of the Soret coefficient is independent of the magnitude of the temperature gradient imposed up to 14 K/cm. The Soret coefficients obtained for mixtures of toluene and n-hexane differ from the values obtained by previous investigators with a thermogravitational column method, but they are in good agreement with the results recently obtained by Kohler and Muller with a forced Rayleigh scattering method. For mixtures of ethanol and water, our Soret coefficients agree with the results obtained earlier by Kolodner et al., also with an optical beam-bending technique.