The pH of biological systems is important for the activity of enzymes, and abnormal cellular pH is related to many diseases. Spatial and temporal modulation of pH with light will be useful for studying the pH effects on enzymatic functions and disease mechanisms and may lead to new drug delivery and therapeutic methods. However, the pH of biological systems is maintained by pH buffers, which implies that only temporary pH change (pH pulse) can be induced in an open system. A key fundamental problem is whether a photoinduced pH pulse can be strong and long enough to generate a significant effect. In this work, a photoinduced pH pulse in a micrometer hydrophilic film in PBS buffer has been demonstrated. The thin film was made of an metastable-state photoacid (mPAH) polymer. It is an open system that allows exchange of protons. A quick release of the protons from the mPAHs and the proton exchange between the film and PBS resulted in a pH pulse generated by moderate visible-light irradiation. The magnitude of the pulse is 1.4-1.9 units with maximum pH change occurring after similar to 18 s of the irradiation. Since the mPAH is a reversible photoacid, the pH pulse could be repeatedly generated after the photoacid recovered in the dark. This work shows that photochemical modulation of pH is possible even in buffered solutions.