The La + O and La + O-2 chemiionization reactions have been investigated with quantum chemical methods. For La + O-2(X-3 Sigma(g)) and La + O-2(a(1)Delta(g)), the chemiionization reaction La + O-2 -> LaO2+ + e(-) has been shown to be endothermic and does not contribute to the experimental chemielectron spectra. For the La + O-2 (X-3 Sigma(g)) reaction conditions, chemielectrons are produced by La + O-2 -> 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(P-3) and La + O-2(X-3 Sigma(g)) reaction conditions. For La + O-2(a(1)Delta(g)), a chemielectron band with higher electron kinetic energy than that obtained from La + O-2 (X-3 Sigma(g)) is observed. This is attributed to production of O(ID) from the reaction La + O-2(a(1)Delta(g)) - LaO + O(ID), followed by chemiionization via the reaction La + O(D-1) -> 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(-) chermionization reactions.