Very compact cores (n in the range 10^5-10^7cm-3) have been observed in
absorption in the ISOCAM maps of the rho Ophiuchi main cloud, at 6.75 and 15
mic (Abergel et al.  1996).  These cores allow to map the spatial variations
of the infrared extinction properties of dust grains with the density and the
local radiation field.  Recent sub-mm observations on the balloon-borne
PRONAOS-SPM have demonstrated that such cores are very cold, therefore the
grain properties should drastically change inside these cores.  We plane to
analyse the variations of the silicate extinction features and also to look
for the presence of a continum in the extinction curve from 5 to 16.5 mic.