An 1:8 RSFQ demultiplexer (DEMUX) and an 8:1 RSFQ multiplexer (MUX) were designed, simulated and optimized to operate at about 20 GHz. Both the inputs and outputs of the DEMUX are complementary dual-rail signals. The basic 2-bit DEMUX module is a self-clocked dual-rail T flip-flop. This DEMUX has a simpler structure than an earlier version of a data-driven-self-timed (DDST) DEMUX developed in our laboratory; however its simulated dc bias margin is (-20%, +20%), which is lower than the more complex device (-29%, +29%). The MUX takes lower data rate single-rail inputs and combines them sequentially into a single output. The complementary output is recovered from the output itself to facilitate interfacing to other dual-rail circuits on chip. The calculated dc bias margin is (-30 %, +28%). Circuit functionality is verified for the 2-bit MUX and the 2-bit DEMUX at low speed. An on-chip high-speed test system is designed to evaluate operation of the MUX and DEMUX at 20 GHz. The circuits are fabricated using a 1 kA/cm/sup 2/ niobium process at both UCB and HYPRES.