We propose ISO observations to derive the near-infrared energy
 distribution of High-Mass X-ray Binaries (HMXBs) in order to
 study the velocity and density structure of the disrupted
 stellar wind. Infrared free-free and free-bound emission is
 formed in the inner wind regions where the major part of the
 acceleration takes place. This radiation is modelled rather
 straightforwardly and is relatively independent of parameters
 such as the level of ionization and the temperature of the
 wind that are difficult to determine. X-ray, UV and optical
 observations have suggested that the stellar wind leaving the
 massive companion is highly distorted in the direction of the
 X-ray source. An extended Stromgren zone surrounds the X-ray
 source and trails the compact star in its orbit. Inside this
 zone the radiative acceleration drops leading to the formation
 of a large and dense shock front in the wind. In general, the
 stellar wind is expected to be inhomogeneous, which might explain
 the observed X-ray variability, and focussed towards the X-ray
 source. Infrared observations at different orbital phases will
 enable us to determine the detailed wind structure and the
 deviations with respect to undisturbed stellar winds. These
 observations will provide better insight in the hydrodynamics of
 radiation-driven winds in this extreme regime and the nature
 of the interactive accretion proces. The predicted IR flux of
 these systems is just below the sensitivity limit of IRAS, but
 easily feasible with ISO. The large extinction in the direction
 of these systems makes the infrared wavelength domain uniquely
 suited to study their stellar-wind structure. We will apply for
 coordinated observations with X-ray, UV and optical telescopes.
 The scientific goals of this proposal can be reached with ISO
 observations only.