The calibration of a dual fibre optical probe was experimentally investigated in a circulating fluidized bed. The particle volume fraction registered by the optical probe was compared to pressure drop measurements for a range of operating conditions. A theoretical calibration theory was applied to the probe signal to obtain the local and instantaneous particle volume fraction. It was found that glare points on the particles and the beam length from the probe to the particle determines the curvature of the calibration function to a large extent. (c) 2004 Elsevier B.V. All rights reserved.