Police believe there could be a connection between the theft of a car, and the attempted theft of another, in Kings Langley on Friday.
Hertfordshire Constabulary said that a Mercedes Benz was stolen after a suspect, or suspects, forced their way through the back door of a house in Love Lane before stealing jewellery and the keys to the car.
Giles Cooper, from Hertfordshire Constabulary, said: "They left through the front of the property and took the car with them."
A second house, in Watford Road, was also broken into when offenders smashed a window at the rear of the property before taking the keys to a Mercedes Benz.
However, the battery on this car was flat so thieves were unable to take it.
Police said the theft in Love Lane and the attempted theft in Watford Road were "possibly connected."
The incidents are believed to have happened between 8am and 6pm and Hertfordshire Constabulary is appealing for witnesses and information.