Lanthanide-based metal-organic frameworks (Ln-MOFs) are fascinating because of their versatile coordination geometry, unique luminescent and magnetic properties, and possible high framework stability to water. We synthesized nanosized europium-based MOF (Eu-MOF) particles and investigated the photoinduced electron transfer between the excited Eu-MOF nanoparticles and various organic compounds, such as aromatic sulfides and amines. From the time-resolved emission measurements, the bimolecular quenching rate constants of luminescence from the Eu(3+) ions in the MOF framework by electron donors were determined and explained in terms of the Marcus theory of electron-transfer reactions. Furthermore, spatially resolved emission quenching images obtained by confocal fluorescence microscopy revealed that small (large) quencher molecules quickly (slowly) and homogeneously (inhomogeneously) penetrate microsized Eu-MOF crystals. These observations led us confidently to assume the possibility that Eu-MOFs work as a size-selective photocatalyst for the one-electron oxidation of organic compounds.