The effective Young's moduli of co-continuous Al-Al2O3 composites over the 5-97 vol% Al2O3 composition range were experimentally measured and compared with theoretical composite modulus values predicted using the methods of Ravichandran, Tuchinskii, Hashin-Shtrikman, and the effective medium approximation (EMA). The influence of phase morphology and the modulus ratio (E-1/E-2) of the constituent phases on the resulting experimental and calculated Young's modulus is discussed. For two-phased co-continuous composites with a modulus ratio greater than 5, the EMA, with an appropriate microstructural shape factor, was the most consistent method for approximating the composite Young's modulus over the entire composition range.