Storage tanks, a lorry and a trailer were all seized in the raids
About 12,000 gallons (55,000 litres) of fuel has been seized and eight men arrested as part of an investigation into a suspected £3m tax fraud.
HM Revenue and Customs (HMRC) and police raided properties in Merseyside, Manchester and Northern Ireland.
Storage tanks, computers, money and a lorry and trailer were seized.
The HMRC said the fuel was "heavy hydrocarbon oil"
Three men from Merseyside, one from Manchester, two from County Armagh, one from County Down and one from County Antrim were arrested.
The men taken into custody are aged between 34 and 72.
A HMRC spokeswoman said the raids in Knowsley, Liverpool, Manchester and St Helens were at a business and four "domestic addresses", while those in Northern Ireland were at residential properties.
She said that the fuel seized was "heavy hydrocarbon oil capable of running a derv (diesel) engine".
