In previous letters, the constraints necessary for the elimination of i.s.i. in both binary and multilevel f.s.k. were discussed. It is now shown that a system with simultaneous phase and frequency modulation is possible, both modulations being detected with zero i.s.i. In terms of energy per bit required at the receiver input for a given error rate, a 4-phase 2-frequency system is shown to be superior to an equivalent 8-state multiphase or combined phase/amplitude system.