The measured optical depth at 9.7 μm, τ_9.7, as a function of the J − K colour excess, E(J − K), for diffuse sightlines (upper panel) and molecular sightlines (lower panel). The dots represent data from Chiar et al. (2007, and references therein) and the crosses represent our measurements. The dashed line is the correlation between τ_9.7 and E(J − K) found for the diffuse ISM (Chiar et al. 2007). The source indicated by the arrow is SSTc2d_J182852.7+002824. This source is remarkable, since it is the only source with E(J − K) > 2 that is still in agreement with the diffuse ISM correlation, probably due to the effects of ices on the estimated band strength.