Wave-like gas-solid flow in a horizontal pipe has been investigated experimentally. The aim of the investigation was to develop a non-intrusive measuring technique for monitoring the transition from a dilute phase flow to wave-like flow and to measure the properties of wave-like flow. When a gas-solid mixture flows in a wave-like manner though the pipeline of a pneumatic conveying system the solids concentration varies, both axially and radially, with time. As a result, the pressure measured at any location will fluctuate and the difference between measuring points, both axially and radially, can be used to determine the nature of the flow. Previous workers have concentrated on individual pressure measurements and axial pressure differences. In this work the radial pressure difference has been examined. This has been compared with axial pressure measurements and those obtained using capacitance sensors. These techniques have been used to determine both wavelength and velocity. A three-dimensonal numerical model, which is based on the two-fluid theory, was also used to obtain a better understanding of the flow field characteristics.