Stereoscopic particle image velocimetry (Stereo-PIV) was used as the tool to observe the turbulence characteristics in a gas cyclone. In the cylindrical and conical parts of the cyclone, intensive fluctuation occurs in the inner quasi-forced vortex, especially in its core where the precessing vortex core dominates. In the dust hopper, strong turbulence is observed at the interface between the downward flow and the upward flow, as well as in the centerline of the cyclone. Turbulence intensity in the tangential, axial, and radial directions and the Reynolds stresses are seen to be anisotropic: this anisotropy provides the evidence of more appropriateness of the Reynolds stress model (RSM) than the standard k-epsilon model, and the renormalization-group k-epsilon model for numerical simulations in gas cyclones. Due to flow instability and back-mixing caused by the turbulence, separated particles could disperse into and be re-entrained by the upward flow from the bin to degrade the separation efficiency of the cyclone. (c) 2006 American Institute of Chemical Engineers.