One of the most significant problems related to pressurised pulverised coal combustion (PPCC) is the separation of the fly ash at operating conditions (gas temperatures of 1400 degrees C and a pressure of (up to) 16 bar). To determine the efficiency of an advanced multi-stage separation device, which was made to deliver clean gas (upstream of a turbine) with a required maximum particle size smaller than 3 pin and a particle mass concentration of less than 3 mg/Nm(3), a novel scattered light particle sizer (A 150-HDT) was developed. The device allows for measurements of a sample flow at a pressure of up to 16 bar and (to date) at temperatures of up to 450 degrees C (however, with further enhancements, up to 600 degrees C to 800 degrees C is expected). Several series of measurements using this device have been performed successfully at a PPCC test site, and results of two series - as well as results gained from LPI measurements - will be presented, which show a sufficiently high efficiency of the multi-stage separator in the supermicron size range for the last measurement series. The novel measurement device proved to be capable of measuring the required low particle concentrations with sufficient precision even under these demanding conditions, and the detected clean gas concentrations indicate a successful removal of the fly ash particles, which fulfils the requirements for feed gases entering a gas turbine. (C) 2007 Elsevier B.V. All rights reserved.