Composite durability can be influenced by varying the properties of the fiber/matrix interphase region. One method to modifY the properties of this interphase is through the application of a sizing to the carbon fiber. Recent work at Virginia Tech has shown that polymer-modified interphases can lead to increases by as much as two orders of magnitude in notched fatigue lifetime. In the present work, an apparatus was constructed to uniformly coat carbon fiber tow with water-soluble and dispersible polymers. Few such devices have been developed for use in academic settings because of the processing complexities presented by the aqueous coating system. Due to the high surface tensions of the aqueous solutions, fiber clumping and heterogeneous sizing deposition were major bottlenecks. Our novel process utilizes high tensions, high spreading, and low line speeds to accomplish the sizing step. Each processing independent variable can be continuously monitored and controlled which allowed for statistical correlation to the sizing level and uniformity. The sizing process was shown to satisfy three criteria for quality. 1. The average sizing level or weight percent on the final fiber can be readily controlled to achieve typical target values. 2. Filament clumping as a result of cohesion between corresponding filaments is kept to a minimum. 3. The sizing process must produce fiber with a consistent level of polymer sizing. In addition, characterization techniques for the sized fiber were developed. Pyrolysis in a high temperature nitrogen furnace was developed as a precise technique to ascertain the quantitative sizing level on the carbon fiber. SEM and ESCA were utilized to determine fiber clumping and sizing homogeneity. The sizing process along with the statistical process model and the characterization techniques allow for the precise development of optimal interphase materials that are tailored to meet the performance requirements of the composite consumer. / Master of Science