AbstractExperiments were carried out with cylindrical thermal‐diffusion columns in which the separation chambers were sufficiently narrow so that curvature effects might be ignored. For both open and closed operation, results were obtained for the two binary systems benzene‐carbon tetrachloride and n‐heptane–benzene as well as the ternary mixture n‐heptane–methylcyclo‐hexane‐toluene. For the binary systems, correlations are given in terms of dimensionless combinations which arise when the equations of conservation are placed in dimensionless form. The thermal‐diffusion coefficients used in the correlations are literature values determined in steady‐state–cell (no convection) measurements.