A detailed examination of the interferometric determination of penetrant concentration profiles during longitudinal penetration of micromolecular liquids in glassy polymer films, nonrigidly clamped between glass plates, is presented. Penetration of methylene chloride and methyl alcohol (representing strong and relatively weak swelling agents, respectively) into cellulose acetate films, characterized, respectively, by a degree of acetylation of 2.45 (CA-2.45) and 2.0 (CA-2.0) acetyl groups per monomer unit, was studied, and the importance of certain factors, which should be taken into account in order to obtain reliable concentration profiles, is demonstrated. In particular, conversion of the interferometrically determined optical path difference profiles to concentration profiles requires taking account of (i) the deviation from linearity of the refractive index-concentration relation, due to the filling of the excess free volume of the polymer by the penetrant and (ii) swelling of the clamped polymer film in the thickness direction, with particular attention to deviations from uniform specimen thickness. In the example of strong swelling agent penetration (CA-2.45/MC), no detectable increase in film thickness over that of the original dry film was detected, as a result of the fact that the polymer becomes sufficiently plasticized to deform plastically along the penetration axis under the clamping pressure. During weak swelling agent penetration (CA-2.0/MA), on the other hand, significant swelling in the thickness direction was observed, because the polymer is less plasticised and can undergo plastic deformation as above only to a limited extent. In this case, it is important to guard against deviations from uniform specimen thickness. The relevant strain birefringence profiles provided significant information which helped to establish the above picture. The calculated concentration profiles are discussed in the light of the measured penetration kinetics and in relation to the magnitude of the clamping pressure.