This paper extends a previous work on direct-sequence code-division multiple access (DS-CDMA) to multicarrier (MC) CDMA systems. New results on the error performance of a family of multiuser detectors for MC-CDMA are obtained using the replica method, which was originally developed in statistical physics. The central result is the "decoupling principle", namely, an MC-CDMA channel with frequency-selective fading followed by a generic multiuser detection front end can be decoupled into a bank of single-user fading channels in the large-system limit. Thus conditioned on one's fading coefficients, each user essentially experiences an equivalent single-user Gaussian channel with a degradation in the signal-to-noise ratio (SNR) in lieu of multiaccess interference. A set of joint equations is identified that determine the degradation for each user, known as the multiuser efficiency. The result applies to arbitrary input distribution and SNRs, and to optimal multiuser detection, as well as various other suboptimal schemes.