A composite membrane system, constituted by two cellulose membranes and an aqueous solution of poly(ethylene oxide) (liquid membrane), was used to investigate the transport phenomena occurring when a thermal difference was applied. At the steady state, pressures were measured (ca. 6.0×105 dyn cm−2) far higher than those reached in ordinary thermoosmosis experiments. In order to explain these unusual effects a model was proposed in which the contribution of the polymeric liquid membrane was taken into account. The thermodynamics of irreversible processes was used to derive the composite membrane phenomenological equations containing both the classical thermoosmosis and the thermal diffusion as particular cases. In this paper the observed phenomena were indicated as “pseudothermoosmosis.” The flow of matter, phenomenological coefficients and heat of transport were also measured. For the activation energy a value of 3.91 kcal mol−1 was found, indicating the diffusive nature of the solvent flow through the composite membrane.