  We present a survey of the formaldehyde emission in nine class 0 protostars
obtained with the IRAM 30m and the JCMT millimeter telescopes. Using a detailed
radiative transfer code of the envelopes surrounding the protostars, we show
that all but one of the observed objects show an inner warm evaporation region
where the formaldehyde is much more abundant (up to three orders of magnitude)
than in the outer cold part. The largest inner formaldehyde abundances are
associated with the sources having the lowest submillimetric to bolometric
luminosity ratio, i.e. with sources closer to the class I border. These
abundances are compared with predictions from recent models of hot core
chemistry.
