The optical beam deflection method was applied for exploring chemical reactions occurring at a single particle. A probe beam from a diode laser was introduced into a microscope, and focused to the vicinity of a particle. When a chemical reaction occurs at the surface of the particle, concentration gradients exist in the vicinity of the particle due to the diffusion of chemical species involved in the reaction. Ion exchange reactions occurring at a single particle of ion exchangers were used as model systems. The results showed that the tendency of change in the OBD signal with time is opposite of that seen in the reverse ion exchange reactions, both theoretically and experimentally. Also, the experimental results indicated that the concentration gradients changed with time in the diffusion layer around the particles, implying that the actual reaction-rate determining step was not the particle diffusion but film diffusion. (c) 2006 Elsevier B.V. All rights reserved.