A new fibre optic calibration method for determination of solid concentration in circulating fluidized beds (CFB) has been proposed. The probe was calibrated by a new in situ technique, based on the measurement of solid mass flux and velocity in the centre of a CFB column. The calibration curve obtained by this method was compared with data from an ex situ calibration set up. It was shown that the in situ method gave more reliable results for concentrations below 3% that are the usual range of concentrations in this field. Local solid concentrations were measured in the CFB for different mass fluxes. The integration of the profiles yielded average concentrations close to those calculated from pressure drop in the column with good accuracy. Local solid concentrations measured by this optical technique are compared to those obtained from the division of upward and downward fluxes and velocities. The results obtained from these two techniques are in satisfactory agreement.