An equation correlating work of adhesion (W(a)) with Young’s modulus and tensile strength of silica-filled polymer composites is derived. It shows that the logarithms of Young’s modulus and tensile strength are inversely proportional to W(a). Fourier transform infrared (FT. i.r.) results of the composites show that the silica interphase thickness increases with increased W(a)h values (the hydrogen bond component of W(a)). The logarithmic correlation between interphase thickness and W(a) is similar to that found for both Young’s modulus and tensile strength. These similarities suggest that W(a) can be used to quantify interfacial bonding. Our study further shows that the composite with the lowest W(a) value follows the Guth Smallwood equation for predicting Young’s modulus. However, as the interphase layer becomes thicker (increased W(a) value), Young’s modulus increases more than predicted from the Guth-Smallwood equation. Thus, an extension of the Guth Smallwood equation is introduced to account for the effect of W(a) on the Young’s modulus value.