In this paper, complementary code keying (CCK) transmission over frequency-selective fading channels is investigated. Since CCK can be interpreted as a block code, an appropriate vector signal model is introduced. Based on pairwise error probabilities, we derive a performance approximation for optimum detection in general frequency-selective Ricean fading channels and compare the theoretical results with simulations. Because optimum maximum-likelihood sequence estimation (MLSE) is not practical for large channel delay spreads, minimum mean-squared error decision-feedback equalization (MMSE-DFE) is employed. Two different DFE approaches are considered here: (a) the conventional scalar DFE and (b) a block-based DFE optimized for the underlying vector signal model. Furthermore, a reduced-state sequence estimation (RSSE) algorithm is designed for CCK. Numerical results are given for fixed channels and common indoor fading channel profiles.