A broad theoretical and experimental investigation of retention in thermal field-flow fractionation is reported here. Equations connecting retention parameters with underlying thermal diffusion constants are reviewed, and new equations are developed to account for the distortion of the flow profile caused by a variable viscosity. Parameters investigated experimentally include channel width, solute molecular weight, channel temperature drop, cold-wall temperature, sample size, and solvent effects. All but the last two of these experimental studies showed good conformity with theoretical predictions. No theory exists for the prediction of sample-size or solvent effects. With regard to the latter, experimental results show that a variety of organic solvents are very effective, at roughly equal levels, in providing retention, while aqueous solvents are generally ineffective.