  We used the KOSMA 3m telescope to map the core 7'x5' of the Galactic massive
star forming region W3Main in the two fine structure lines of atomic carbon and
four mid-J transitions of CO and 13CO. The maps are centered on the luminous
infrared source IRS5 for which we obtained ISO/LWS data comprising four high-J
CO transitions, CII, and OI at 63 and 145 micron. In combination with a KAO map
of integrated line intensities of CII (Howe et al. 1991), this data set allows
to study the physical structure of the molecular cloud interface regions where
the occurence of carbon is believed to change from C+ to C0, and to CO. The
molecular gas in W3Main is warmed by the far ultraviolet (FUV) field created by
more than a dozen OB stars. Detailed modelling shows that most of the observed
line intensity ratios and absolute intensities are consistent with a clumpy
photon dominated region (PDR) of a few hundred unresolved clumps per 0.84pc
beam, filling between 3 and 9% of the volume, with a typical clump radius of
0.025pc (2.2"), and typical mass of 0.44Msun. The high-excitation lines of CO
stem from a 100-200K layer, as also the CI lines. The bulk of the gas mass is
however at lower temperatures.
