We describe the synthesis and photochemistry of coumarin-functionalized silica nanoparticles, which were prepared utilizing 7[3-(triethoxysilyl)propanyloxy]coumarin (TPC) to attach coumarin as a photoactive group to the silica nanoparticle surface. The nanoparticle size and morphology were investigated by scanning electron microscopy, atomic force microscopy, and dynamic light scattering. The diameter of the spherical nanoparticles was determined by all three methods to be about 40 nm. The surface functionalization was characterized in the bulk by xi-potential measurements and on the single-nanoparticle level by electrostatic force microscopy, where the difference in surface potential between TPC-modified and unmodified silica nanoparticles is measured. The degree of surface functionalization was determined by thermogravimetric analysis (TGA), and a theoretical limit of about 23 000 coumarin entities per nanoparticle was calculated. The photochemistry, and its reversibility, of the nanoparticle-attached coumarin entities was found to be quite different from the coumarin photochemistry in solution or on flat surfaces. Photodimerization with light of 355 nm and photocleavage with light of 254, 266, and 280 nm were analyzed by absorption and fluorescence spectroscopy. Following several cycles of photodimerization and photocleavage showed that the absorption change at 320 nm decreases from cycle to cycle. The coumarin layer on the nanoparticles was proven to be unchanged by TGA. The apparent loss of absorption change is due to the formation of interlinked nanoparticles during the dimerization-cleavage cycles. Because the coumarin groups on the inside of the obtained nanoparticle clusters are inaccessible to light, the amount of "uncleavable" dicoumarins increases, thus lowering the obtainable absorption change from cycle to cycle.