Depletion of small dust grains in the early stages of molecule formation

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.