In April 1999. the Control of Major Accident Hazards Regulations 1999 (COMAH), came into force in Great Britain, implementing the requirements of the Seveso 11 Directive 96/82/EC, The Competent Authority (CA) for the COMAH regulations in England and Wales comprises the Health and Safety Executive (HSE) working jointly with the Environment Agency (EA) (and in Scotland, the HSE working with the Scottish Environment Protection Agency (SEPA)). This arrangement reflects the requirements to ensure the protection of both persons and the environment. The COMAH regulations superseded the Control of Industrial Major Accidents Hazard Regulations 1984 (CIMAH), which were implemented by HSE alone. The first COMAH major accident with environmental consequences was in July 1999 at a chemicals storage facility operated by Tees Storage, at Seal Sands, Middlesbrough. Sixteen (16) tonnes of sodium cyanide solution leaked from a storage tank into the ground, polluting the groundwater and the River Tees. This incident occurred only four months after the regulations came into force and provided an early test for the workings of the CA. It was one of the 13 COMAH major accidents which were reported to the European Commission in the first year of operation of the regulations (1999/2000). Another COMAH major accident with environmental consequences occurred in October 2000 at a waste management and treatment facility operated by Cleansing Services Group (CSG), at Sandhurst in Gloucestershire. A major fire started in the early hours of the morning, during a severe storm, and approximately 60 local residents were evacuated. The tire service were unable to gain access to fight the fire and the police set up a 'Gold Control' to manage the incident. Three days later the River Severn burst its banks flooding the site and the local residents were evacuated for a second time. The CA deployed significant resources to investigate the incident, make the site safe and handle the public concern about health impacts. This paper examines the causes of the incidents, their investigation and the regulatory actions taken by the CA. It also considers some of the longer term lessons learned.