A strength of materials and elasticity approach was used in this study to determine the stiffness of fiber-reinforced composites, by taking into account the concept of boundary interphase. Theoretical expressions for longitudinal transverse and shear moduli, as well as for longitudinal Poisson's ratio, were derived by use of this model. Results derived from these expressions were compared with others, to observe the discrepancies and evaluate their validity. (C) 2005 Wiley Periodicals, Inc.