The compositional changes taking place during the wet spinning of acrylic fibers from an aqueous sodium thiocyanate solvent were investigated. The composition of the fibers diverted from the precipitation bath after various immersion times was determined gravimetrically, while fiber diameters were imaged to ascertain volumetric changes with time. The kinetics of phase separation were approximated using light transmission and video techniques applied to acrylic films. For coagulation into water at 20 and 40 degrees C and into 15% aqueous NaSCN at 20 degrees C, a greater influx of the nonsolvent to outflow of the solvent was recorded at short timescales. Unexpectedly, both the outflow of the solvent and nonsolvent against the concentration gradient was noted at longer timescales, suggested by the light transmission data to be after the primary phase separation. The consequent reduction in filament diameter, hence, the volume, is discussed in terms of a coarsening mechanism, whereby the mobile polymer lean phase has a route away from the filament into the bath during polymer coarsening. Finally, the compositional changes are plotted on a phase diagram for the system as trajectories into the two-phase region. The polymer-coarsening effect renders the interpretation at longer timescales uncertain.