The importance of O₃ pulse duration for encapsulation of organic light emitting diodes (OLEDs) with ultra thin inorganic atomic layer deposited Al₂O₃ layers is demonstrated for deposition temperatures of 50 °C. X-ray reflectivity (XRR) measurements show that O₃ pulse durations longer than 15 s produce dense and thin Al₂O ₃ layers. Correspondingly, black spot growth is not observed in OLEDs encapsulated with such layers during 91 days of aging under ambient conditions. This implies that XRR can be used as a tool for process optimization of OLED encapsulation layers leading to devices with long lifetimes.