Control over phase separation and morphology is critical to optimal function in polymer optoelectronic devices. Here, two fully conjugated oligomeric compatibilizers are introduced, and their effect on the phase separation of blends of poly(9,9'-dioctylfluorene-co-benzo-thiadiazole) (F8BT) with poly(9,9'-dioctylfluorene-co-bis-N,N'-(4,butylphenyl)bis-N,N'-phenyl-1,4-phenylenediamine) (PFB) are reported. AFM and STXM analysis demonstrate that the addition of compatibilizer altered the size and relative composition of phase-separated domains formed during spin-casting. Small structural differences between the two compatibilizers brought about significantly different morphological changes to the blends, suggesting that further development of compatibilizer structure could enable enhanced control toward desired blend film morphologies.