The mechanical and viscoelastic properties of unidirectional vinyl-ester carbon fiber composites were investigated. A cooperativity analysis of the composites was performed on storage (E') and loss modulus (E") master curves obtained from dynamic mechanical analysis. The temperature sensitivity of the horizontal logarithmic shift factors (log a(T)), cooperativity, obtained from E' data, was found to vary with the sizing used to pretreat the carbon fibers. The observed variations in the experimental trends in cooperativity with fiber sizing for the composite materials were found to deviate significantly from theory. However, the trends in tensile and apparent shear strength of the composites matched the observed qualitative trends in viscoelastic cooperativity obtained from the storage modulus master curves. These results may suggest that the viscoelastic and ultimate mechanical properties of a composite material are related. However, several inconsistencies were observed when comparing the E" and E' data which should also be considered in the interpretation of the experimental results.