We propose to study the winds of massive stars in the high
 mass-loss phase of their evolution, i.e. in stages from O- to
 A-supergiants. The intention is to use the unique spectroscopic
 and photometric capability of ISO to measure accurate strengths
 and profiles of H and He lines formed in stellar winds,
 and the IR-continuum which is also formed in the wind.
 By means of detailed NLTE radiative transfer and line formation
 techniques it will be possible to determine rates of mass-loss
 and stratification of wind velocity fields much more precisely
 than hitherto possible. (The present best estimates of the
 mass loss rates of A and B-stars have an uncertainty of one order
 of magnitude
    The results of this study will have important repercussions
 for our understanding of massive star evolution. They will
 also provide a crucial test for the theory of radiation driven
 winds, which is normally applied to predict stellar wind
 properties as function of stellar parameters. Last but not
 least, the results will be used to calibrate the wind
 momentum-luminosity relation of blue supergiants which has
 been found very recently and which has great potential for
 extragalactic distance determinations.