The effects of an inhomogeneous interphase zone on the effective bulk modulus of particulate composites containing large concentrations of inclusions are analyzed. The composite is modeled as a suspension of elastic homogeneous hollow spherical particles in a continuous elastic matrix. An interphase zone surrounding inclusions where the matrix material has elastic moduli with radial variation that asymptotically assume a constant value far away from particles is then modeled. The related elastic problem of a single inclusion in a finite matrix subjected to a spherically symmetric load is considered and a closed-form solution in terms of hypergeometric functions is determined. This analytical solution is then employed to derive an explicit expression for the effective bulk modulus. A detailed parametric analysis is finally performed to investigate the influence of the geometric characteristics and elastic properties of the graded interphase zone for composites containing voids or solid/hollow inclusions.