The temperature and ionization state of the circumstellar matter in WR/OB colliding wind binaries are influenced by a variety of physical processes and their mutual interplay. Under discussion are shock heating, heat transfer by electrons or X-ray photons, instabilities of the interaction zone, radiative braking, and the ionizing stellar radiation fields. The complex entanglement of all these different effects prevents a fully consistent model so far. The review, therefore, rather adresses some selected single processes and their consequences.

The hot post shock zones which are part of the interaction zone contribute significantly to observed X-ray spectra and light curves. Following largely the literature, we discuss their properties. Shocks are responsible for the heating of these zones. Pre-shock conditions and the shock geometry significantly influence the strength of the shock. They are influenced by various effects. Several authors have addressed the question of heat transfer. They have shown that heat transfer is basically able to efficiently pre-heat wind material before it gets shocked, leading to a further dependence of pre- and post-shock states. Wind inhomogeneities, whose existence has been proposed in various papers, probably will have a large effect on the interaction zone as well. Instabilities of the interaction zone have been under investigation for quite some time by now. They affect the generation and distribution of hot post shock matter. Cooling processes are crucial in this connection, especially the question of which processes are to be considered and under which conditions.

Various authors have argued that the cold, high density part of the interaction contributes to the optical and UV spectrum. This part is subject to a variety of influences as well. Instabilities were mentioned already. Heat transfer certainly is efficient but due to the high densities is also in strong competition with radiative cooling. In contrast to the high temperature zones the stellar radiation field here may be of importance as well. Finally, we touch the suggestions of some authors who have brought this part of the interaction zone in connection with dust formation.