The investigation presented examines the compressive strengths, Xc, of thermoplastic-toughened epoxy composites with fiber surface chemistry altered by various levels of surface treatments and two distinctly different sizings. The thermoplastic, polyvinylpyrrolidone (PVP) size improved the IITRI Xc by 51% over an unreacted bisphenol-A epoxy size for a 100% surface-treated fiber. It is important to point out that effects were produced with the sizings making up less than 1% (by weight) of the composite. Thus, appropriate selection of sizings may present an effective and economical means for tailoring composite material performance. It is suggested that properties other than interfacial shear strength may be important in the description of composite compression strength, e.g., interphase modulus and fracture resistance.