  We present the results of an analysis of ultraviolet observations of T Tauri
Stars (TTS). By analysing emission measures taken from the literature we derive
rates of ionizing photons from the chromospheres of 5 classical TTS in the
range ~10^41-10^44 photons/s, although these values are subject to large
uncertainties. We propose that the HeII/CIV line ratio can be used as a
reddening-independent indicator of the hardness of the ultraviolet spectrum
emitted by TTS. By studying this line ratio in a much larger sample of objects
we find evidence for an ionizing flux which does not decrease, and may even
increase, as TTS evolve. This implies that a significant fraction of the
ionizing flux from TTS is not powered by the accretion of disc material onto
the central object, and we discuss the significance of this result and its
implications for models of disc evolution. The presence of a significant
ionizing flux in the later stages of circumstellar disc evolution provides an
important new constraint on disc photoevaporation models.
