In situ electron microscopy, tomography, photoluminescence, and X-ray absorption spectroscopy were utilized to monitor and explain the formation and growth of internal pores within ZnO nanorods. Careful examination using electron microscopy and tomography indicate that nanosized internal pores start appearing within the individual solution-grown ZnO nanorods upon exposure to 200 ℃. The pore volume growth rate is proportional to the heat treatment time, indicating that the process is diffusion controlled, akin to a reverse Ostwald ripening-like process. A manageable pore growth rate of 1.4–4.4 nm³·min^-1 was observed at 540 ℃, suggesting that the effective control over internal porosity can be achieved by carefully controlling the heat-treatment profile. Mechanistic studies using X-ray absorption spectroscopy indicated that the pore formation is linked to the significant reduction of the number of zinc vacancies after heat treatment. An optimum condition exists where most of the native surface defects are removed, while the bulk defects are contained within the internal pores. It is also demon-strated that the internal porosity can be exploited to improve the visible light absorption of ZnO. A combination of the lower defect density and improved light absorption of the heat-treated ZnO films thus lead to an increase in the photo-electrochemical response of more than 20× compared to that of the as-grown ZnO.