Block copolymer lithography is being actively researched as a patterning technology to supplement existing 193nm immersion photolithography and enable the semiconductor industry to keep pace with Moore's law. Block copolymer films generally require post-deposition annealing, and the annealing process itself is critical. Solvent annealing is a newer alternative to the incumbent thermal annealing technique and overcomes many of the limitations of thermal annealing, however the fundamentals of solvent annealing have yet to be understood in part because existing solvent annealing techniques are quite primitive. A new controlled co-solvent vapor annealing system was designed and assembled which provides precise and independent control over solvent vapor activity. The phase behavior of a cylinder-forming poly(styrene)-b-poly(ethylene oxide) block copolymer annealed with toluene and water vapor was investigated for a range of solvent vapor activities. Thin film morphology was characterized using a combination of atomic force microscopy, transmission and scanning electron microscopy, and synchrotron grazing-incidence small-angle X-ray scattering. The thin film morphology is strongly influenced by the solvent removal process, and transformations between ordered phases are possible during this step. The domain spacing of the block copolymer, a key parameter for patterning applications, can be tuned from 25nm to 31nm by adjusting the solvent vapor activities during annealing; this represents an increase of up to 40% relative to the bulk equilibrium domain spacing of 22.6nm. The applicability of this technique to the directed self-assembly of block copolymers was demonstrated by tuning domain spacing to control the number of rows of block copolymer domains which pack into fixed-width graphoepitaxial confinement features.