The magnetic field effects (MFEs) on the photocatalytic degradation of methylene blue (MB) solution over ZnO particles have been studied. A positive MFE was clearly confirmed, while the MFE is decreased with increasing the interval time (settling time) between the preparation of MB solution and the start of photodegradation. It is suggested that one of the key factors of the MFEs is the amount of dissolved oxygen in the MB solution. © The Royal Society of Chemistry 2011.