Tantalum nitride (Ta3N5) and oxynitride (TaON) are promising materials for photoelectrochemical (PEC) water splitting due to near-ideal band gaps and band edge positions. However, the high-temperature ammonolysis process that is usually used to make these materials depends sensitively on the process parameters and specific design of the annealing system, and reproducing highly efficient (oxy)nitride photoanodes from recipes reported by other laboratories has proven to be challenging. To understand and monitor the nitridation process in more detail, we employ an optical absorption spectroscopy technique that allows us to follow the transformation of thin Ta2O5 films in situ at temperatures up to 800 degrees C. Our results show that the incorporation of nitrogen in a dry ammonia atmosphere starts at 575 degrees C and is accompanied by a gradual red-shift of the Ta2O5 absorption edge and an expansion of the lattice due to the larger ionic radius of N-3 relative to O-2. Although coloration of the material due to an N-2p ? Ta-3d transition occurs readily, the films do not show any visible-light PEC activity until the nitrogen concentration is high enough to form a continuous N-2p impurity band. Ta3N5 is found to be the only thermodynamically stable phase between 575 and 800 degrees C, with no traces of TaON. Longer nitridation times result in lower defect concentrations, larger grain sizes, and improved PEC performance. The photocurrent of well-crystallized films is limited by slow water oxidation kinetics. This can be effectively remedied by depositing IrO2 nanoparticles as a water oxidation cocatalyst, which results in external quantum efficiencies of up to 45%. The smaller enhancement of the PEC performance at longer wavelengths reveals that hole transport in Ta3N5 limits the water splitting performance of IrO2-catalyzed Ta3N5 photoanodes.