The Poohbah Lake complex is an Archean undersaturated potassic pluton consisting of porphyritic syenite, malignite, amphibole syenite, and biotite–pyroxenite, intrusive into Couchiching metasediments. Apparent K–Ar ages for biotites from the alkaline rocks fall into two groups, one (2 samples) with ages greater than 2700 m.y., the other (8 samples) forming a tight cluster of ages at 2556 ± 36 m.y. This latter group defines a K–Ar isochron age of 2706 ± 23 m.y. The data are interpreted to imply that the isochron age and the ca. 2700 m.y. apparent ages record the cooling age of the complex and that the majority of samples were subjected to a later thermal event which resulted in loss of equal amounts of Ar from each sample, this event being correlated with epeirogenic strike-slip faulting. Algoman granite and pegmatite (3 samples), which on geological evidence was emplaced later than the alkaline rocks, gives apparent K–Ar ages of 2550–2630 m.y. Algoman mica samples plot on the alkaline rock isochron giving 2702 ± 24 m.y. as a minimum age for plutonism in this area.