which is between the conventional liquid fluidization and the dilutephase liquid transport regime. They also characterized this regime with axial uniformity. This was later confirmed by several other researchers (Kuramoto et al., 1998;Zheng et al., 1999a) using different types of particles and various measurement techniques.With respect to the radial flow structure, Liang et al. (1 996) reported that the radial non-uniformity can be clearly recognized in the liquid-solids circulating fluidization regime using a conductivity probe.The solids concentration increases from the centre to the wall of the riser. Lately, Zheng et al. (1999b) confirmed the above radial structure by using a fibre-optic probe. At the same time, they pointed out that the radial flow structure is affected significantly by the operating conditions and particle properties. The radial non-uniformity is increased with increasing solids circulation rates but decreasing liquid velocity. At the same average solids holdup, lighter particle systems have a relatively more uniform radial flow structure. Furthermore, they showed that the radial non-uniformity is a unique characteristic of the liquid-solids circulating fluidization regime, which is different from the uniform radial flow structure observed in the conventional liquid-solids fluidization regime. However, the above conclusions are all for the time-average flow structure (the macroflow structure). Studies on the instantaneous flow structure (microflow structure) have not been reported.Knowledge of the microflow structure is important to the design and operation of the liquid-solids circulating fluidized bed reactors. For example, the radial distributions of mass and heat transfer behaviour are closely linked to the fluctuations of the local solids concentration. Liquid and particle residence time distributions also depend on the fluctuations as well as the time-average values of the concentration and velocity of particles and liquid in the riser. All these parameters are the key aspects which affect the design and scale-up of a LSCFB. To gain a better understanding of the local flow behaviour, it is important to examine the instantaneous flow behaviour in addition to the time-averaged flow behaviour. The probability density function can provide an overall picture of local voidage fluctuations and a more quantitative description is reflected in the standard deviation of the local solids holdup about its mean. In the gas-solids circulating fluidized beds, microflow structure has been studied by many researchers through recording time variations of gas and solids flow. For example, Schnitzlein and Weinstein (1988) characterized the flow structure using instantaneous pressure signals.~~ *Author to whom correspondetice may be addressed. E-mail address: zhu@uwo.ca I instantaneous and time-average suspension densities were determined in a 76 mm diameter by 3 m tall liquid-solids circulating fluidized bed riser using a fibre-optic probe. Attempts were made to qualify the microflow structure throu...