Abstract: Effects of supercooling have been studied in a range of basaltic melts by isothermal and constant cooling rate experiments at one-atmosphere, using the wire-loop container method. The nucleation of plagioclase in these melts is systematically controlled by supercooling (−ΔT), time below the liquidus temperature, and initial superheating (+ΔT. The temperature at which the melt is initially superheated prior to supercooling controls the temperature at which crystal nucleation first takes place in a supercooled melt undergoing cooling; the greater the + ΔT the larger the degree of supercooling required prior to nucleation of the liquidus phase during cooling. In all six compositions investigated there are at least two fields in time-temperature space, one in which the liquidus phase always fails to nucleate on supercooling (favoured by small −ΔT) and one (favoured by large −ΔT) in which it always nucleates. These two fields may be separated by another in which the liquidus phase may or may not crystallize. Supercooling phenomena are not restricted to the liquidus phase but can also occur when the melt becomes saturated with subsequent phases. It is shown that the composition of plagioclase varies systematically with −ΔT and it is demonstrated that isothermal supercooling “lines” can be used with a relatively high degree of accuracy to predict when nucleation of the liquidus phase will take place during constant cooling rate experiments.