The envelope technique is a powerful analytical tool already known in spectrophotometry, but only recently introduced in ellipsometry. We will develop its formalism for tanpsi in weakly absorbing interference films and see how data extraction can be greatly simplified. The envelope technique permits to separate the extraction of the refractive index of the film from the thickness. Therefore, it is ideal for those moderately thick films affected by multivalued solutions in conventional regressions, typically in polymer film preparations. We present some experimental results for a photolithographic resin deposited on glass. (C) 2004 Elsevier B.V. All rights reserved.