The La + O and La + O2 chemiionization reactions have been investigated with quantum chemical methods. For La + O2(X3Σg) and La + O2(a1Δg), the chemiionization reaction La + O2 → LaO2+ + 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(X3Σg) reaction conditions. For La + O2(a1Δg), a chemielectron band with higher electron kinetic energy than that obtained from La + O2(X3Σg) is observed. This is attributed to production of O(1D) from the reaction La + O2(a1Δg) → 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.