Summary: The mode of occurrence of uraninite, pitchblende and coffinite at five localities in south-west England is described. Chemical and isotopic analyses of concentrates containing these minerals provide the following 206Pb/238U ages (My.): uraninite, South Crofty, Cornwall, 277 ± 10; uraninite, Geevor, Cornwall, 223 ± 5; pitcbblende-coffinite, King's Wood, Devon, 206 ± 5; pitchblende, Wheal Bray, Cornwall, 165 ± 4; pitchblende, South Terras, Cornwall, 474 ± 2; coffinite, Geevor, Cornwall, c. 45 (chemical analysis only). These results are compatible with geolo-gical and mineralogical evidence. Consideration of these and other results indicates that there were at least three periods of uranium mineralization in south-west England, at c. 290 My., c. 225 My., and c. 50 My. Other intermediate ages may also be valid but further work is required to substantiate these.