Various methods exist to determine the temperature of the central stars of planetary nebulae. The most important of these are the 4 following methods: 1)Zanstra method 2)Energy balance method 3)NLTE study of the central star spectrum 4)Modelling the degree of nebular ionization The first three methods are known to be dependent on various uncertain assumptions concerning the central star energy distribution, the nebular optical depth, and/or the physical state of the central star atmosphere. They therefore give conflicting and controversial temperatures. The fourth method, for which we plan to use ISO observations, does not require any assumptions regarding either the optical depth or the spectral energy distribution. In fact, it is able to determine this latter quantity if a sufficient range of ionization stages are observed. The difficulty is that in the optical and IUE ultraviolet only 'low' ionization stages are seen, the highest being the difficult to observe NeV state which requires 97 ev to produce it. Thus not enough information is available to distinguish temperatures above 100000 K. The ISO SWS is able to observe lines of much higher ionization potential, up to 303 ev. Using these observations it will not only be possible to accurately determine high temperatures, but it will be possible to determine the actual spectral distribution of the central star.