At the 2001 census, the Cynon Valley had a population of 63,512,[4] 12.1% of whom were Welsh speakers. In common with some of the other South Wales Valleys, Cynon Valley had a high percentage of Welsh speakers until the early 20th century.[citation needed]
Former district
From 1974 to 1996, the borough of Cynon Valley was one of thirty-seven districts of Wales. The district was formed from the Aberdare and Mountain Ash urban districts, the parish of Rhigos from Neath Rural District and the parish of Penderyn from Brecknockshire.[5] It was one of six districts of Mid Glamorgan, and in 1996 was merged into the larger unitary authority of Rhondda Cynon Taf. Throughout the council's existence, the Labour Party held a majority of the seats on the council.[6] The council was based at Rock Grounds on High Street in Aberdare, which was built in 1938 for one of the council's predecessors, Aberdare Urban District Council.[7][8][9]
In 1999, a Welsh Assembly constituency with the same boundaries was formed.
In 2010, the constituency was redefined as consisting of 15 electoral divisions of the county borough of Rhondda Cynon Taf: Aberaman North, Aberaman South, Abercynon, Aberdare East, Aberdare West/Llwydcoed, Cilfynydd, Cwmbach, Glyncoch, Hirwaun, Mountain Ash East, Mountain Ash West, Penrhiwceiber, Pen-y-waun, Rhigos, Ynysybwl.[12]