(1) Two radiocarbon-dated pollen diagrams are presented from high-altitude 'blanket peat' sites in South Wales. Cefn Ffordd, north-west of Rhondda, has a shallow, peaty gleyed soil supporting Molinia bog at 600 m O.D., the highest altitude in Glamorgan. Brecon Beacons has deeper, hagged hill peat at 715 m O.D. on the Brecon Beacons escarpment, 15 km east of Pen-y-Fan. Both sites are on 'moisture-shedding' convexities. (2) The pollen diagrams were divided into phases rather than conventional pollen-assemblage zones. The dates of the boundaries of each phase were interpolated from deposition-rates spanning pairs of radiocarbon-dated horizons. The ecological histories of the two sites are discussed and compared with reference to this time-scale. (3) The peat began to form at different times. At the Brecon Beacons site it dates from the (late) Neolithic, and at Cefn Ffordd from the (early) Bronze Age. Neither date corresponds with most published statements concerning the initiation of Welsh blanket peats. (4) At the higher-altitude site the peat formed earlier. The pre-peat vegetation at this site appears to have been open hazel woodland. Peat at the lower altitude site was probably initiated under Callunetum, although hazel woodland probably grew there earlier in the Post-Glacial. Prehistoric man may have influenced the initiation of these blanket peats as there is some evidence in horizons close to the mineral-peat transition for his activity in the area.