This paper, first of a two-paper series, analyzes the experimental results to clarify the effects of gas leakage on the separation performance of a cyclone. Experiments were carried out in a cold model of a cyclone. Varied parameters were inlet gas velocity, inlet solid concentration, and gas leakage velocity. In the experimental range, the pressure drops over a cyclone showed small variation with the increase in gas leakage velocity. When the gas leakage fraction exceeded 4%, experiments showed that the separation performance deteriorated significantly. Gas leakage re-entrained separated fine particles in the standpipe into the cyclone, indicated by a significant decrease of the separation efficiency for fine particles. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.