  ISOCAM allowed the observation of diffuse interstellar regions where the
transition from atomic to molecular gas takes place. In this contribution we
report on spectacular variations of small dust grains abundance in such regions
observed with ISOCAM. In the four cirrus-like clouds observed, we note a
systematic disappearance of small dust grains in regions where CO emission
starts to appear. We suggest that the variations of the small dust grain
abundance observed here are related to the damping of the gas turbulent motions
that favors the coagulation of dust through grain-grain collisions.
