The Gully Oolite at St Lythams, Glamorgan, is extensively dolomitized towards its base. The dolomite is non-ferroan and is both replacive and void-filling in nature. δ18O varies from −6.06%0 to −2.96%0 (PDB) and δ13C from +2.32%0 to 3.55%0 (PDB) with a marked covariation of δ18O and δ13C. Dolomitization is believed to have occurred within a mixing-zone environment at salinities up to 50% of that of seawater. The variation of dolomite isotopic composition versus depth in the sediment is explained in terms of successive upwards migration of the mixing-zone and associated multiple phases of dolomitization. The tops of these successive mixing-zones coincide with evidence of sediment emergence, and the presence of dolomite intraclasts implies that dolomitization occurred at shallow depths within the sediment. This suggests dolomitization occurred in the absence of an overlying meteoric-phreatic lens. The high degree of dolomitization at St Lythams as opposed to that occurring in modern mixing-zones is related to: (1) longer duration of the mixing-zone environment in the sediment; (2) the proximity of the mixing-zone to the sediment surface; (3) possible chemical differences between Carboniferous and modern seawater, particularly with respect to pco2.