Probe electrospray ionization mass spectrometry (PESI-MS) has been demonstrated to be a useful in situ and online analytical technique for monitoring of various reactions. In this work, PESI-MS with a surface-modified probe was adopted and applied to in situ monitoring of photocatalytic reactions. Typical reactions of semiconductor photocatalysts, namely TiO₂, SnO₂, WO₃, SiC and ZnS catalyzed methylene blue (MB) and brilliant green (BG) degradation, were selected to demonstrate the potential of PESI-MS to monitor heterogeneous photocatalytic reactions occurring in suspensions. Surface modification of the probe ensures increased wettability during the whole monitoring process. PESI-MS could provide continuous sampling and real-time MS results without time-consuming and cumbersome sample pretreatments. This method has other merits including good reproducibility and stability (time scale > 60 min), convenience of operation, low sample consumption, high time resolution and high tolerance to suspended photocatalyst particles. Time-resolved mass spectra and ion chromatograms of every chemical species e.g. the substrate and reactive intermediates could be obtained, which is helpful for a better understanding of the photocatalytic reaction process. Thus, PESI-MS could be a versatile analytical technique for in situ photocatalytic reaction analysis and could be an alternative means for the evaluation of photocatalyst performance.