The aim of this paper is to calculate the electrophoretic mobility, mu(D), of an isolated spherical porous particle in an alternating electric field. Formulas for mu(D) are obtained for the case where the double-layer thickness is much less than the pore radius and for the case where the zeta potential is small. The mobility spectrum has a similar form in both cases, a form that is very different from that of a solid particle of the same size and density. The most striking difference is that the porous particle remains mobile and moves in phase with the field at high frequencies while the solid particle mobility approaches zero with a limiting phase lag of 45 degrees. The formulas obtained here can be used to determine the size and zeta potential from the measured mobility spectrum for dilute suspensions of porous particles. They could be used, for example, to monitor the growth of particles and the drop in zeta potential during a flocculation process.