Abstract In the present study, the buffering effect of magnetite nanoparticles (Fe3O4) dispersed in an aqueous solution on the local pH value is investigated. It manifests itself in the fact that when some amount of acid or base is added to the solution then the solution near the nanoparticles surface becomes, respectively, less acidic and less alkaline than it is expected. It is the result of both the local electrostatic field, which represents the electric double layer at the surface of magnetic nanoparticles and the magnetic field around the nanoparticles. The magnetite nanoparticles exhibit very low toxicity and they are becoming increasingly important for new biomedical applications related to their effects on chemical reactions in body tissues and cells. The question arises, how strong are these effects at the nanoscale? The strength of the buffering property of magnetite nanoparticles is investigated both theoretically and experimentally by the direct measurement of the local pH value of a magnetic nanoparticles suspension. The theoretical model is based on stochastic equations describing the ions diffusing in the neighborhood of the electric double layer of the magnetic material. The electric double layer is modeled with the help of the Poisson–Boltzmann model. It is directly shown that both the electrostatic field and the magnetic field are responsible for the observed local changes of the pH value with respect to the bulk pH value.