  We have investigated the ISM of the I Zw 1 QSO host galaxy with Plateau de
Bure mm-interferometry and high angular resolution near-infrared imaging
spectroscopy. We have detected a circumnuclear gas ring of diameter ~ 1.5''
(1.8 kpc) in its millimetric CO line emission and have mapped the disk and the
spiral arms of the host galaxy in the ^{12}CO(1-0) line at 115 GHz as well as
in the H (1.65 um) and K (2.2 um) band . Combining our new mm- and NIR-data
with available estimates of the radio- and far-infrared contributions to the
nuclear emission, we find strong evidence for a nuclear starburst ring. A
comparison to other sources with nuclear activity indicates that these rings
may be a common phenomenon and contribute a large fraction of the central
luminosity.
