<p>Photocatalytic and photoelectrochemical (PEC) water splitting are considered key technologies associated with the future implementation of artificial photosynthesis. Since the dawn of this research field, various chemical and physicochemical phenomena characteristic of semiconductor/liquid interface have been reported. Meanwhile, in recent studies, photocatalyst and photoelectrode materials are generally being developed under the following assumptions: 1) The basic principles involved in photocatalytic and PEC reactions represent a combination of photovoltaics and electrolyzers on the microscopic scale and can thus be explained separately by analogy to photovoltaic physics and electrochemistry. 2) Effective utilization of the energy of photons for chemical reactions critically depends on the development of superior solid materials, including semiconductor photocatalytic particles and/or cocatalyst nanoparticles. But are these assumptions valid? This review highlights the important contribution of physicochemical phenomena in water, which have tended to be overlooked, during photocatalytic and PEC reactions. The benefits of a buffered aqueous solution during water splitting, which include accelerated proton-related mass transfer and a suppressed local pH gradient in the vicinity of the active sites, are summarized. In addition, a possible interaction between electronic band structures inside the semiconductor photocatalytic materials and transient physicochemical phenomena in water is proposed.</p>