A controlled co-solvent vapor annealing system was designed and constructed to investigate the effects of solvent vapor activity during the rapid ambient quenching process on the morphology of a cylinder-forming poly(styrene)-b-poly (ethylene oxide) (PS-PEO) annealed in toluene and water vapor. A phase transformation from cylinders in the bulk to close-packed spheres in swollen thin films occurred, which was reversed upon quenching with dry nitrogen. Quenching with humidified nitrogen preserved the spherical morphology but could significantly alter domain spacing and reduce long-range order in the dried films under some circumstances. Specifically, long-range order in the quenched films was found to decrease as the quenching humidity decreased from the humidity used during annealing, and the best long-range order was obtained when the humidity remained consistent throughout both annealing and quenching. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 1125-1130