The Mona Complex exposed on the Llŷn peninsula includes the Sarn Complex, which is a suite of variably-deformed, plutonic igneous rocks (originally mapped partly as gneisses) ranging from gabbro to granite, and an inlier of high-grade paragneisses. Geochemical data for samples from the Sarn Complex may be divided into two distinct populations. The Rb-Sr data for samples from these geochemical groups define significantly different ages. The older age, 549 ± 19 Ma, initial 87Sr/86Sr 0.7096 ± 3, is identified as the age of intrusion in late Precambrian to Cambrian times, while a younger age of 458 ± 16 Ma may approximate to the age of a resetting event. The Parwyd paragneisses yield an Rb-Sr whole-rock isochron age of 542 ± 17 Ma, concordant with the age of intrusion of the Sarn Complex, with an initial 87Sr/86Sr of 0.7069 ± 4. For the Sarn Complex, δ18O values range from +9.5 to +12.0 per mil (SMOW), and in the Parwyd metasediments from +9.4 to +13.3 per mil. We conclude that the earliest magmatism and metamorphism in the Mona Complex of Llŷn occurred during late Precambrian to Cambrian times, broadly contemporaneous with similar events in Anglesey. The high initial 87Sr/86Sr and high δ18O values for the Sarn Complex are most simply interpreted as reflecting either a crustal source-region or incorporation of a significant proportion of crustal material (such as the Parwyd gneisses) into a mantle-derived magma.