We propose to obtain complete 2.5-45 micron grating spectra with the SWS of a sample of dusty M-supergiants. In a recent paper we have shown that some M-supergiants display in their mid-IR spectra not only the expected 9.7 micron silicate feature, but also the family of Unidentified InfraRed bands at 7.7, 8.7, 10.5, 11.3 and 12.7 microns. The UIR bands are recognised as diagnostics of regions with carbon-rich chemistries, but from optical spectra and the presence of the silicate dust features M-supergiants are seen to have oxygen- rich chemistries. Therefore the appearance of UIR bands in M-supergiant spectra is entirely unexpected. We concluded that chromospheres around the M-supergiants provide UV photons which photodissociate CO in their stellar winds, yielding free carbon which then builds the Polycyclic Aromatic Hydrocarbons (PAHs) responsible for the UIR bands. Here we will be able to detect the entire family of known UIR bands in a sample of M-supergiants having a wide range of mass-loss rates, in 3 OB associations, in order to better determine the conditions under which the UIR bands do or do not appear in M-supergiant spectra. By determining which of the known bands appears, we hope to be able to characterise the type and size of PAH molecules present around each star.