  We have 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. In combination with a map of
singly ionized carbon (Howe et al. 1991), and FIR fine structure line data
observed by ISO/LWS at the center position, these data sets allow to study in
detail the physical structure of the photon dominated cloud interface regions
(PDRs) where the occurance of carbon changes from CII to CI, and to CO.
