HD168625 is an evolved luminous early-type supergiant (Mbol=-8.6),
 recently found to be surrounded by a dusty nebula most probably
 ejected by the star (Hutsemekers et al.  1994).  It is related to
 the rare Luminous Blue Variable stars (LBVs), which constitute an
 important stage in the evolution of the very massive stars.
 Since these stars are expected to eject material enriched in
 nitrogen at the expense of carbon, the presence in the spectrum
 of HD168625 of Unidentified Infrared emission features at 6.2,
 6.9, and 7.7 micron (UIRs), generally associated with carbon-rich
 objects (Cohen et al.  1986), is puzzling and unique among LBVs
 and related stars.
 We propose here to carry out a detailed study of the
 circumstellar environment of HD168625, i.e.
 1.  to locate the origin of the UIR emission features
     using ISO CAM,
 2.  to derive the C, N, O abundances in the outer wind + nebula
     using the far-infrared fine structure emission lines
     observable with ISO LWS.
 For comparison, we also propose to derive the C, N, O abundances
 for the nearby LBV HD168607, which is of comparable luminosity
 and spectral type, but which is not associated with a nebula.
 These data will provide us with important information on the
 evolutionary status and mass-loss history of both stars, as well
 as on dust formation processes.
 These data will supplement with a particularly interesting object
 the other LBV observations foreseen by different consortia in the
 framework of the Guaranteed Time programme.