We Propose a Simple Method for Accurately Estimating the Solids Circulation Rate in a Closed-Loop Liquid-Solid Circulating Fluidized Bed. the Technique is based on the Use of Radioactive Isotopes for Estimating the Solids Velocity and Volume Fraction Distribution in a Chosen Section of the Circulating Fluidized Bed Loop. the Method is Demonstrated for a Liquid-Solid Circulating Fluidized Bed, and is Readily Applicable in Other Systems Such as Gas-Solids Circulating Fluidized Beds or Closed-Loop Pneumatic and Hydraulic Transport Lines. with Minor Modifications, the Method May Be Used for Industrial Systems as Well. © 2001 Elsevier Science B.V. All Rights Reserved.