The interaction between the fiber and matrix in a fiber‐reinforced material plays an important role in determining the mechanical behavior of the composite. An efficient technique to simultaneously improve fiber‐matrix interfacial shear strength and impact behavior of the composite is to deposit a flexible interlayer onto the fiber. This results in the creation of three bulk phases, the fiber, matrix, and the interlayer and two interphasial regions. A phenomenological model that defines the variation of the fiber‐interlayer interphase and that of the interlayer‐matrix interphase has been developed. In the model, the elastic moduli of these regions vary continuously, so as to bridge the two bulk phases on either side of the interphase. The interaction between the bulk phases is also taken into consideration. The model has the potential for the use of dynamic mechanical analysis to obtain, relatively, adhesion/interaction parameters of different fiber‐interlayer‐matrix systems. These parameters can be used to determine the optimum interlayer thickness for improved toughness and good stress transfer efficiency.