2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/394A steep metallicity dependence of the CO-to-H2 conversion factor X of the form log X = 12.2 - 2.5 log[O]/[H], based on a sample of 20 (extra)galactic determinations, is presented and discussed. The apparently less steep dependence found from millimeter interferometer observations is explained within the context of photon-dominated regions.4 pages, 2 figures, in: "H2 in Space", Cambridge University Press, ed. F. Combes and G. Pineau des ForetsAstrophysicstext