Circulating Fluidised Beds (CFB) are attracting increasing interest for both gas-solid and gas-catalytic reactions, although the operating modes in these two cases are completely different. In modelling CFBs as reactors, the solids residence time is an important parameter. Previous studies mostly assess operations at moderate values of the solids circulation rates (<= 100 kg/m(2) s), whereas gas-catalytic reactions and e.g. biomass pyrolysis require completely different operating conditions. In the current work, Positron Emission Particle Tracking (PEPT) is used to study the movement and population density of particles in the CFB-fiser. The PEPT results can be used to obtain: (i) the vertical particle movement and population density in a cross sectional area of the riser; (ii) the transport gas velocity (U-tr) required in order to operate in a fully established circulation mode; (iii) the overall particle movement mode (core flow versus core/annulus flow); and (iv) the particle slip velocity (U-s). Only in a core flow mode can the particle slip velocity be estimated from the difference between the superficial gas velocity (0 and the particle terminal velocity (U-t). The slip velocity is lower than U- Ut outside the core flow mode. To operate in core flow, the superficial gas velocity should exceed U-tr by approximately 1 m/s and the solids circulation rate should exceed 200 kg/m(2) S. (c) 2007 Elsevier B.V. All rights reserved.