Ultrafine fibers of a pH-responsive triblock copolymer with different diameters, alignment, and microdomain order are prepared via electrospinning and solvent-vapor annealing. Fibrous samples display much quicker response times than the corresponding films owing to the increase in external surface area. The fibrils of annealed, aligned, electrospun fibers show a dramatic apparent "1D" pH-induced size-change during the experiment.