K-Ar ages for the three major intrusive episodes of the Uasilau-Yau Yau intrusive complex have been determined. Group 1 rocks (gabbro and quartz diorite) are at least 30 m.y. old. Group 2 intrusions (quartz diorite, tonalite, and granodiorite), which form the main volume of the complex, were emplaced as a number of comagmatic phases at about 28.4 m.y. Intrusion of group 3 tonalite at about 23.5 m.y. was followed closely by hydrothermal alteration and copper mineralization. Emplacement of quartz-feldspar porphyry took place after mineralization. Igneous activity in the Uasilau-Yau Yau intrusive complex spans a period of at least 6.6 m.y. It is suggested that all porphyry copper mineralization in New Britain may be the result of a late Oligocene igneous event.