U–Pb zircon and (or) titanite or monazite ages have been obtained for five major, undeformed, calc-alkaline plutons of the Appalachians of southeastern Quebec. These are interpreted as ages of crystallization for the Scotstown (384 ± 2 Ma), Lac aux Araignées (383 ± 3 Ma), Winslow (377 ± 7 Ma), Aylmer (375 ± 3 Ma), and Ste-Cécile (374 ± 2 Ma) plutons. Many other titanite samples gave 206Pb/238U dates that are 2–16 Ma younger than the concordant zircon dates from the same samples, and this is probably the result of Pb loss. Variation in 207Pb/206Pb ages of titanite from some samples is attributed to incorrect common Pb correction.Rb–Sr data for the same plutons show considerable isotopic heterogeneity and correspondingly high errors in ages. The isotopic heterogeneity is likely caused by postsolidification metasomatic alteration by host rock fluids. Where the scatter is least (Ste-Cécile), the Rb–Sr age (364 ± 14 Ma) is similar to the U–Pb mineral age (374 ± 2 Ma). The 87Sr/86Sr initial ratios range from 0.7065 to 0.710 and are probably related to the source of the magmas. The relatively high initial ratios and the peraluminous nature of the plutons preclude a significant mantle contribution to the magmas. These undeformed plutons are probably the result of melting of the lower continental crust near the end of crustal thickening caused by compression during the Acadian Orogeny.