 On 1 July 1916 an area with roughly 1,332 inhabitants was moved to Skien, and on 1 July 1920 an area with 437 inhabitants was moved to Porsgrunn.On 1 January 1964 the rest of Gjerpen was incorporated into Skien, along with Solum and the district Valebø.Prior to the merger Gjerpen had a population of 15,300.