  We investigate the relationship between the C18O column density and the
visual extinction in Chamaeleon I and in a part of the Chamaeleon III molecular
cloud. The C18O column densities, N(C18O), are calculated from J=1-0 rotational
line data observed with the SEST telescope. The visual extinctions, Av, are
derived using JHK photometry from the 2MASS survey and the NICER color excess
technique. In contrast with the previous results of Hayakawa et al. (2001), we
find that the average N(C18O) / Av ratios are similar in Cha I and Cha III, and
lie close to values derived for other clouds, i.e. N(C18O) ~ 2 x 10^14 cm^-2
(Av - 2). We find, however, clear deviations from this average relationship
towards individual clumps. Larger than average N(C18O) / Av ratios can be found
in clumps associated with the active star forming region in the northern part
of Ch a I. On the other hand, some regions in the relatively quiescent southern
part of Cha I show smaller than average N(C18O) / Av ratios and also very
shallow proportionality between N(C18O) and Av. The shallow proportionality
suggests that C18O is heavily depleted in these regions. As the degree of
depletion is proportional to the gas density, these regions probably contain
very dense, cold cores, which do not stand out in CO mappings. A comparison
with the dust temperature map derived from the ISO data shows that the most
prominent of the potentially depleted cores indeed coincides with a dust
temperature minimum. It seems therefore feasible to use N(C18O) and Av data
together for identifying old, dense cores in large scale mappings.
