1983–1997: The Borough of Merthyr Tydfil, and the District of Rhymney Valley wards Nos. 5 and 14 to 20.
1997–2010: The County Borough of Merthyr Tydfil, and the Caerphilly County Borough wards of Abertysswg, Darren Valley, Moriah, New Tredegar, Pontlottyn, Tir-Phil, and Twyn Carno.
2010–2024: The Merthyr Tydfil County Borough electoral divisions of Bedlinog, Cyfarthfa, Dowlais, Gurnos, Merthyr Vale, Park, Penydarren, Plymouth, Town, Treharris, and Vaynor, and the Caerphilly County Borough electoral divisions of Darran Valley, Moriah, New Tredegar, Pontlottyn, and Twyn Carno.
The constituency was created for the 1983 general election. Prior to this, Merthyr Tydfil had been in its own eponymous constituency, and Rhymney was in the seat of Ebbw Vale (at the time Michael Foot's constituency), which was abolished at that election.