The La + O and La + O2 chemiionization reactions have been investigated with quantum chemical methods. For La + O2(X 3Σg) and La + O2(a1Δ g), the chemiionization reaction La + O2 → LaO 2+ + e- has been shown to be endothermic and does not contribute to the experimental chemielectron spectra. For the La + O2(X3Σg) reaction conditions, chemielectrons are produced by La + O2 -LaO + O, followed by La + O → LaO+ + e-. This is supported by the same chemielectron band, arising from La + O → LaO+ + e-, being observed from both the La + O(3P) and La + O2(X 3Σg) reaction conditions. For La + O 2(a1 Δg), a chemielectron band with higher electron kinetic energy than that obtained from La + O2(X 3Σg) is observed. This is attributed to production of O(1D) from the reaction La + O2(a1Δ g) → LaO + O(1D), followed by chemiionization via the reaction La + O(1D) → LaO+ + e-. Potential energy curves are computed for a number of states of LaO, LaO* and LaO+ to establish mechanisms for the observed La + O → LaO+ + e- chemiionization reactions. © 2008 American Chemical Society.