The coded performance of a slow-frequency hopped differential PSK (SFH/DPSK) system in the presence of both tone jamming and additive white Gaussian noise (AWGN) is studied. The error correlation due to DPSK demodulation and the effect of tone jamming is considered when evaluating the block and decoded error probabilities. The decoded error probabilities in some special cases are computed. The effect of interleaving on system performance is addressed under various conditions. A nearly optimum code rate for a Reed-Solomon code is determined for systems employing interleaving techniques. The effect of partial-band tone jamming is demonstrated.