The Heavy Gas Dispersion Trials (HGDT) project consisted of an extensive programme of field trials on the dispersion of heavy gas clouds at ground level in the atmosphere. The programme was organised by the Health and Safety Executive as a cooperatively funded project with a total of 38 other organisations. Planning commenced in 1979 and the trials were performed between 1982 and 1984 on a site at Thorney Island, West Sussex. The conduct of the trials was contracted to NMI Ltd. The primary objective of the trials was the acquisition of reliable data at large scale with which to test the validity of mathematical and physical models. The basic concept of the trials was the study of the dispersion of fixed-volume, isothermal clouds under a variety of conditions. The trials were divided into two separately funded phases. In Phase I the dispersion was over uniform, unobstructed ground and comprised a total of 16 trials. In Phase H the dispersion took place in the presence of one of three different types of obstacle and comprised a total of 10 trials. This paper describes the design of the Phase I trials and summarises the results obtained.