The present work aims at coorelating the surface characteristics of a "working" catalyst under the conditions of the oxidative coupling of methane (OCM) with mechanistic aspects of the reaction deduced from transient experiments focused on the total oxidation pathway. Several lanthana samples with variable surface area and morphology are tested with a view to identifying the exact nature of OCM-specific active sites. The lanthana surface, characterized by XPS, TEM, XRD, FTIR, and in situ DRIFT spectroscopy, appears to be mostly decarbonated and dehydroxylated under reaction conditions. Low coordination sites localized on step edges are related to the total oxidation pathway, while basic sites associated with oxygen vacancies enable the oxygen activation leading to the initial methane activation. Structure sensitivity effects are explained within this proposal of differentiated sites on a typical OCM catalyst.