The catalytic converters from four cars were stolen during a car boot sale at Huntingdon Racecourse.
The cars were parked at various locations throughout the course just off the A14 at Brampton on Sunday (May 17).
Owners reported the thefts after discovering their converters were missing when they went to leave the event.
The catalytic converters fitted to all modern cars are designed to reduce toxic emissions. To do this, they use a variety of precious metals, most commonly platinum, but also palladium and rhodium.
They have significant scrap values, making them desirable to thieves.
Anyone with any information regarding this incident should call 101 and quote the crime reference number CF0194770515.