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Stable film morphology is critical for long‐term high performance organic light‐emitting diodes (OLEDs). Neutron reflectometry (NR) is used to study the out‐of‐plane structure of blended thin films and multilayer structures comprising evaporated small molecules. It is found that as‐prepared blended films of fac‐tris(2‐phenylpyridyl)iridium(III) [Ir(ppy)₃] in 4,4′‐bis(N‐carbazolyl)biphenyl (CBP) are uniformly mixed, but the occurrence of phase separation upon thermal annealing is dependent on the blend ratio. Films comprised of the ratio of 6 wt% of Ir(ppy)₃ in CBP typically used in OLEDs are found to phase separate with moderate heating while a higher weight percent mixture (12 wt%) is found to be stable. Furthermore, it is found that thermal annealing of a multilayer film comprised of typical layers found in efficient devices ([tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA)/Ir(ppy)₃:CBP/bathocuproine (BCP)]) causes the …