The Rutherford backscattering (RBS) method has been employed to study the incorporation of oxygen into MgB 2 films during their fabrication by pulsed-laser deposition (PLD). A series of MgB 2 thin film samples were analyzed, including two films produced in situ on Al 2 O 3 c substrates (with higher T c and lower T c ) with an on-axis geometry, one film produced in situ with an off-axis geometry, and one film produced ex situ, with a bulk-like T c . The amount of oxygen detected by RBS, which is stable in the form of MgO, appears to be correlated with the T c of the films, the higher the T c the lower the oxygen content. The superconducting properties of the thin films are discussed in the context of the RBS results.Index Terms-MgB 2 thin films, Rutherford backscatter.