Magnetic field effect (MFE) on heterogeneous photocatalysis is investigated, using mostly ZnO powder and methylene blue (MB) solution. Reproducibility with errors less than 2% is obtained with our newly-developed in-situ measurement system. Multiple parameters are involved in MFE phenomena. MFE magnitude depends on magnetic field intensity, while other parameters are also important, including: initial MB concentration, settling time of solution, and level/state of dissolved oxygen (DO). Short-range-order, or very-short-range-order diffusion in the Helmholtz layer just outside powder appears responsible for MFE. Accordingly, powder surface conditions, e.g. H2O, CO2 adsorption, significantly influence MFE, and magnetic adsorption appears correlated. Temperature-dependent MFE and magneto-hydrodynamic effect on photocatalytic silver reduction/deposition are also observed for the first time. A model is proposed, whereby, in the volume in close proximate to the powder, electrostatic potential and the corresponding magnetic flux would be perturbed, resulting in the appearance of a net Lorentz force on paramagnetic DO, which is essentially responsible for various MFE phenomena (OANS model). DO could be involved with (i) formation of DO-dye complex, (ii) modifying powder adsorption layer, (iii) scavenging rate of excited electrons, (iv) spin chemistry involving singlet-triplet conversion, and (v) manifestation of magneto-hydrodynamic effect. Simultaneous multiple mechanisms are suggested for MFE. (C) 2015 Elsevier B.V. All rights reserved.