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.