In this paper, reduced-state sequence estimation (RSSE) designed for complementary code keying (CCK) transmission over frequency-selective fading channels is investigated. An Ungerboeck-like set partitioning of the multidimensional CCK symbol constellation is given. For the proposed RSSE algorithm based on this partitioning, numerical results are provided for a typical indoor wireless channel. A comparison is made with optimum detection and suboptimum decision-feedback equalization (DFE), respectively.