Doping the hole transport layer (HTL) of organic light emitting devices (OLEDs) was found to increase device operational stability. To this effect, the role of 5,6,11,12-tetraphenylnaphthacene (rubrene), a widely dopant for HTLs, in increasing OLED stability has been widely investigated. However, significant disagreements between various explanations for the increased stability, ranging from rubrene being a charge injection promoter, to its being a charge trap, still exist. We conducted an in-depth study on the influence of rubrene doping of HTL on device stability. The study was carried out on OLEDs of structure: indium-tin-oxide (ITO) anode/N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) HTL / tris(8-hydroxyquinoline) aluminum (AlQ3) electron transport layer / Mg:Ag cathode, in which different portions of the HTL were doped with rubrene. Compared to undoped devices, stability of OLEDs in which HTL doping was limited to only a thin interfacial layer at either the ITO or AlQ3 interface was essentially the same, whereas, stability of OLEDs in which a substantial portion of the HTL was doped was about an order of magnitude higher, and approached that of devices where the whole HTL was doped. In addition, for a fixed thickness of the doped portion, device stability was found to be essentially independent of the thickness of the undoped portion. The results demonstrate that increasing OLEDs stability by means of doping the HTL is associated with changes in bulk HTL hole transport properties rather than interfacial properties, and is consistent with OLED degradation mechanism based on instability of cationic AlQ3 species.