Police have warned drivers they could have their vehicles seized if they are found to be using stolen red diesel.
It follows three raids on yards in and around the Borders in recent weeks.
About 3,000 litres of regular diesel were taken in the first incident, in Hawick, between 5 and 9 November.
Some 200 litres of red diesel were taken from a tank in Greenlaw on the same dates while 2,000 litres were stolen from a yard near the A701 at Penicuik in Midlothian on 10 November.
Lothian and Borders Police said that where significant volumes of fuel were stolen it would require the use of a pump and a large vehicle.
Any sightings of vehicles acting suspiciously at farms and industrial premises should be reported to the police, who said they were stepping up their own patrols.
They have also warned that anyone offered red diesel for use in a normal road vehicle should be aware that it is breaking the law to use it.
The vehicle can be seized and a significant financial penalty imposed.
Police also conduct "dip sampling" of vehicle fuel tanks on behalf of HM Revenue and Customs right across the region.
Red diesel is normally sold to farmers, construction companies and factories to run machinery not used on public roads.
The fuel - which is coloured red to prevent its use by private motorists - is taxed at a lower level compared to the normal diesel levy.