A new correlation has been developed for the purpose of evaluating the effective drag coefficient of gas-solid systems of ascending flow. In the proposed expression, the effects of particle acceleration in the acceleration zone and Basset force have been considered. The radioactive particle tracking measurement technique (RPT) was used to obtain a dynamic picture of particle trajectory in the system. The Cartesian coordinates (x, y and z) of the radiotracer were registered every 10 ms and other useful variables, such as particle velocity, particle velocity fluctuation and acceleration, were calculated. The effects of particle velocity, acceleration and Basset force on the measurement of the drag coefficient were investigated in the internal circulating fluidized bed (ICFB). The experiments were carried out using sand and alumina particles in an ICFB unit with a 1 m riser length and a 0.052 m riser diameter. The gas velocity range studied was between 2 and 12 m/s. The most common correlations for calculating the drag coefficient were reviewed and compared to the one developed in this work. Numerical analysis of the one-dimensional two-phase flow model demonstrates that the drag coefficient proposed here is in good agreement with experimental data and covers a variety of operating conditions (G(s) = 29, 50 and 240 kg/m(2) s; U = 4.2, 5.8 and 8.5 m/s; D = 0.05, 0.2 and 0.4 m ID). (c) 2006 Elsevier Ltd. All rights reserved.