An experimental study is reported of the flow and thermal fields produced by injecting a plane jet of warm water down one wall of a vertical passage of rectangular cross section into a slowly moving upward stream of cooler water. Flow visualisation and particle image velocimetry techniques were used to obtain pictures of the flow. This was found to be turbulent and non-steady under certain conditions. Detailed measurements of local mean velocity, turbulence quantities and temperature were made using a laser Doppler anemometry system and a traversable rake of thermocouples. In the regions where the warm water encountered the cooler water the variation of temperature with time was highly intermittent. As Richardson number was increased, the influence of the buoyancy, opposing the flow, had the effect of stabilising it and restricting the downward penetration of the jet and its lateral spread. In the case of the experiments with the higher values of Richardson number a very concentrated shear layer was formed at the interface between the two flow streams. The turbulence was strongly modified in this region, its intensity peaked and the turbulent shear stress changed sign.