Its wards were split between Blaenau Gwent and Rhymney, and Merthyr Tydfil and Aberdare.

[3] 1983–1997: The Borough of Merthyr Tydfil, and the District of Rhymney Valley wards Nos.

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 seat was bordered by the constituencies of Blaenau Gwent, Brecon and Radnorshire, Caerphilly, Cynon Valley, and Islwyn.