A variety of flow regimes may be observed in dilute phase pneumatic transport of particulates. As the gas flowrate is reduced or the solids flowrate increased, particles may settle on the bottom of the pipe, forming either a stagnant layer or slowly moving dunes. Solids tend to deposit in inclined sections before they deposit in horizontal sections. The change in flow regime resulting from solids segregation leads to such problems as flow instabilities and very long residence times for some particles. Maintaining a consistent operation and product quality requires rapid detection of any changes in flow regime. In many industrial applications, the installation of intrusive sensors is undesirable. The objective of this study was to ' develop reliable flow regime detection through the on-line analysis of signals from non-invasive acoustic sensors, using various solid particles at pipe inclinations of 0 degrees, 15 degrees and 25 degrees with respect to the horizontal direction. Non-intrusive microphones were used to record acoustic emissions generated by solids flow through a 0.1 m diameter, stainless steel, pneumatic transport pipe, at various solids fluxes and superficial gas velocities. Measurements were recorded on the side and top of the pipe, to record the flow of solids as they hit and are reflected from the pipe walls. To confirm the flow regimes, a section of clear acrylic pipe allowed for visual analysis of the flow. Two flow regimes were observed: dilute phase flow and conveying over settled solids. Cycle or frequency analysis of the acoustic measurements recorded on the side and top of the pipe provided reliable, on-line detection of these flow regimes. (c) 2006 Elsevier B.V. All rights reserved.