  X-ray line profiles represent a new way of studying the winds of massive
stars. In particular, they enable us to probe in detail the wind-wind collision
in colliding wind binaries, providing new insights into the structure and
dynamics of the X-ray-emitting regions. We present the key results of new
analyses of high-resolution Chandra X-ray spectra of two important colliding
wind systems, Gamma Velorum and WR140. The lines of Gamma Vel are essentially
unshifted from their rest wavelengths, which we suggest is evidence of a wide
shock opening angle, indicative of sudden radiative braking. The widths of the
lines of WR140 are correlated with ionization potential, implying
non-equilibrium ionization. The implications of these results for the radio
emission from these systems are discussed, as are some of the future directions
for X-ray line profile modelling of colliding wind binaries.
