Circulating Fluidized Beds (CFB) are attracting increasing interest for both gas-solid and gas-catalytic reactions, although operating modes differ completely. To model the reactor, the knowledge of the solids flow in the riser is an important parameter. The design of CFB-riser reactors requires the knowledge of the solids residence time, thus necessitating the study of the particle movement within the riser. Previous studies have mainly focused upon risers operating at lower values of the solids circulation flux, and have distinguished mainly between an S-type and exponential solids hold-up profile in the riser. The present paper used Positron Emission Particle Tracking of a radioactively labelled single particle. The movement of the tracer was monitored in "real time" and determined (i) the particle upwards and downwards velocities; (ii) the population densities of these particles in the cross sectional area of the riser; (iii) the solids flow pattern near the distributor and L-valve; (iv) the existence of 5 different solids hold-up regimes in the riser and the associated (U, G) conditions whereby these different regimes prevail. To operate in a dominant core flow mode, as preferred for reactions where a strict control of the residence time is needed, the superficial gas velocity in the riser should exceed UTR by approx. 1 m/s, whereas G should exceed approximately 100 kg/m2s. To operate in a core-annulus mode, possibly with a BFB at the bottom of the riser, lower G-values need to be applied.