AbstractA technique is described for the measurement of the heat of immersion for polyethylene with a 2 . 10−3J accuracy. For the studied systems, the heat of mixing is positive and due to solubility of the liquid in the amorphous phase of the polymer. The dissolved fraction is estimated from the heat of mixing according to Flory's theory. Measurement of the surface energy from single crystals indicates that this surface constituted of the folds of the chains behaves as an amorphous phase. Last, the heat of immersion is compared to the value calculated from the contact angle. If the solubility of the liquid is very small, the agreement is good. The case of water is studied: the negative value of the heat of immersion ΔHiproceeds from a change in water structure and a decrease of the enthalpy due to the contact with apolar compounds. This value allows to estimate the solubility of water in polyethyle