Improving the quantum efficiency (QE) for photocatalytic water splitting for solar H₂ production is a key research challenge. Reported quantum efficiencies are to date relatively modest. Besides developing new materials absorbing more visible light, increasing the photogeneration and utilization of the charge carriers is of particular interest. It has been reported that under some conditions 90 % of photoelectrons and photoholes recombine within 10 ns in TiO₂ colloids,which would make it very difficult to achieve high efficiency for water splitting as the reactive steps would have to occur on a timescale faster than this. Measuring charge recombination processes and in particular reactions between water and charge carriers in candidate photocatalysts are therefore important steps in evaluating the materials and understanding mechanisms that underlie water splitting by inorganic materials. In the work, we confirm the spectral fingerprint of photoholes and photoelectrons in nanocrystlline TiO₂ (nc-TiO₂) films, and then systematically study the dynamics of photoholes under weak laser excitation down to levels below one photon absorbed per nanoparticle using transient absorption spectra. Furthermore, for the first time we present evidence that four holes are required to produce one molecule of oxygen. The kinetics of water oxidation for O₂ production is also studied in order to explore the timescale of O₂ evolution and to elucidate the rate-determining step in water splitting. Finally we use this new information to suggest some factors that must be addressed to maximise the efficiency of photocatalytic water splitting in both TiO₂ and other materials.