  Maps of the J=2-1 12CO emission from the SbII galaxy NGC 7331 show a
low-contrast ring at a radius of about 3.5 kpc. There is no evidence for a
pronounced central hole in the CO distribution as claimed by others. The
molecular ring is just outside the radius of peak emission from warm dust, but
coincides with the peak of colder dust emission. Various 12CO and 13CO
transitions have been observed from three positions including the center, which
was also observed in the 492 GHz transition. The line measurements have been
modelled by emission from a clumpy mixture of low-density molecular gas at
about T(kin) = 10 K and high-density molecular gas at temperatures of 10 K and
20 K. The CO to H2 conversion factor in NGC 7331 is lower than that in the
Milky Way, and lowest in the center of NGC 7331. The total interstellar gas
mass is dominated by molecular hydrogen in the bulge and in the ring, and by
atomic hydrogen outside the ring. Total hydrogen mass densities in the ring are
about twice those in the bulge. Total gas to dynamic mass ratios increase from
1% in the bulge to 3% outside the ring. The bulge molecular gas may have
originated in mass loss from bulge stars, in which case the molecular ring is
probably the consequence of evacuation efficiency decreases at the outer bulge
edge.
