Organic light-emitting devices (OLEDs) based on end-capped sexithiophene or aluminium-quinolate (Alq 3) with aluminium and ITO contacts were prepared by vapour deposition in high-vacuum (10 −6 mbar). Current–voltage ( I– V), capacitance–voltage ( C– V) and impedance measurements were performed in situ without breaking the vacuum. The OLEDs were then deliberately exposed to oxygen or air. For both materials, the I– V curves of pristine OLEDs are non-rectifying and show significant anomalies at about ±5 V. The capacities are found to be voltage- and frequency-independent. Whereas doping with dry oxygen has only small influence on the electrical properties, doping with air yields a rectifying I– V behaviour without anomalies, but at significantly reduced currents. This is attributed to a decrease in carrier mobility. The capacitance of the Alq 3-OLEDs remains voltage- and frequency-independent, whereas for thiophene-based OLEDs, a “Schottky-like” capacitance with a net carrier concentration of about N eff≈10 17 cm 3 is observed.