In situ grazing-incidence small-angle X-ray scattering experiments on thin films of block copolymers during annealing in neutral solvent vapors are reported. By removing the solvent in a controlled manner, the period of the microphase separated morphology is found to increase with increasing block copolymer concentration in a power law manner with an exponent similar to 2/3. By venting the systems at different rates during the solvent removal process, kinetically arresting the system, the period of the microphase separated morphology in the dried film can be varied.