The ISO galactic survey provides images of the inner disk of the Galaxy in two broad filters (around 7 and 15 μm) over some 15 square degrees, away from the brightest star forming regions. A catalogue of infrared dark clouds has been derived through a multiresolution analysis of these images. Most of them are condensed cores of large molecular clouds, several kpc away from the Sun, seen in absorption in front of the diffuse galactic emission. These cores are internally structured. The 7 to 15 μm contrast ratio is 0.75 ± 0.15 for the clouds located away from the Galactic Centre (|l| > 1°) and 1.05 ± 0.15 for the clouds closest to the Galactic Centre (|l| < 1°, |b| < 0.2°). Using a simple absorption model, we derive a 7 to 15 μm opacity ratio equal to 0.7 ± 0.1 and estimate the opacity of a few objects at 15 μm in the range 1 to 4.