The OH-initiated heterogeneous oxidation of solid methyl beta-D-glucopyranoside nanoparticles (a cellulose oligomer surrogate) is studied in an atmospheric pressure gas flow reactor coupled to an aerosol mass spectrometer. The decay of the solid reactant relative concentration is measured as a function of OH exposure over a wide range of ambient relative humidities (RHs). The kinetic traces display an initial fast exponential decay followed by a slower decay. For long OH exposure, the fraction of a particle that reacts decreases from 90% at RH = 30% to 60% at RH = 20% and to 40% at RH = 10%. A computational model based on the diffusion and reaction of the radical, monosaccharide, and water is developed in order to further examine the experimental data. The model parameters and validity are discussed on the basis of previous literature data. The experimental data are consistent with a diffusion-controlled heterogeneous oxidation. These findings are discussed toward a better understanding of mass transport in semisolid organic material and their effect on chemical change, in particular during the thermal transformation of cellulosic materials to useful chemicals.