Thirty volunteers are expected to take the plunge into the icy waters of the River Thames to raise funds for charity.
Some of them are set to complete the sponsored swim wearing fancy dress costumes over their diving suits.
The event, which takes place on Sunday, has been organised by members of the Swindon branch of the British Sub-Aqua Club, in aid of the Royal National Lifeboat Institution.
The swim starts at 10am from the riverbank close to Inglesham Church, near Lechlade, and finishes at St John's Lock.
It will take participants, ranging from 20-somethings to those in their 70s, about an hour to complete the course.
The Swindon branch of the British Sub-Aqua Club has raised thousands of pounds for the RNLI over the past 25 years.
Nationally, several inshore lifeboats have been purchased through club fundraising efforts.
The start of the swim will be signposted off to the left of the A361 between Inglesham village and Lechlade.
The finishing line, St John's Lock, is located east of Lechlade off the A417.
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