An optical probe has been developed for measurements of solids volume-fraction in high-temperature fluidized beds. The probe consists of two optical fibers: one for exposing the bed particles to monochrome laser light, and the other to receive the light reflected by the particles (backscatter principle). Optical filtering and amplitude modulation of the light source reduce the effect of background radiation. The probe was used in an electrically heated bubbling fluidized bed without combustion, in a cold circulating fluidized bed (CFB) and in the transport zone of a CFB boiler. The results from the electrically heated bed showed that the probe was capable of detecting bubbles at temperatures up to 800 degreesC (maximum operating temperature of the unit). Thus, the optical filtering and modulation techniques were sufficient to eliminate the effects of background radiation. The CFB boiler results (850 degreesC) gave a wall-layer thickness in agreement with corresponding results from a momentum probe and previous solids-flux measurements. Statistical analysis of the optical probe signals showed that the downward solids flux at the wall might consist of clusters but their solids volume-fraction is not uniform. Instead, there is a continuous variation in solids volume-fraction with time, and cluster definitions suggested in literature cannot be applied to the conditions investigated.