This proposal is a inevitable extension of the work that has been started with HLAMERS.HOTSTARS to follow the important stages of hot supergiant evolution in the HRD and to calibrate the Wind-Momentum luminosity relationship properly. 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 we will be possible to determine rates of mass-loss and stratification of wind velocity fields much more precisely than hitherto possible. 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 Relationship (WLR) of blue supergiants which has been found very recently and which has great potential for extragalactic distance determinations with an accuracy rivalling that of the Period-Luminosty relationship of Cepheids.