The occurrence of hydrogen sulphide in the environment, the reasons for concern about the levels which may be present, particularly on off-shore installations, and methods for determining the gas in air are reviewed. The original aim of this project was to explain the reasons for the variations in the levels of naturally occurring hydrogen sulphide over the estuary of the Don at Aberdeen. The monitoring work required a reliable method: as the published methods were all problematical, much effort was expended in improving the Ethylene Blue method and investigating the range of trapping solutions. The final optimised method has been published (Analyst 1988, 113, 1635). An alternative method for standardising H2S in air gas mixes has also been developed and used on commercial samples. Microbiological studies have shown that sulphate-reducing bacteria were not present in the sediments of the Don estuary in 1988 (but were present in sediments from the Forth estuary) and further, that the Don sediments would not support growth of SRB's, which in part explains why there has been effectively no release of H2S from the estuary in recent years. Attempts have been made to correlate the H2S emission events with changes in river water quality over the last ten years, but no obvious clues have become evident.