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Transmission lines are fundamental elements in millimeter-wave circuits. In this paper, on-chip coupled transmission lines, fabricated in a commercial 0.18 ??m complementary metal-oxide semiconductor process, have been modeled, based on measured 50 GHz four-port scattering-parameters. The two-port open-short deembedding technique and thru deembedding method were successfully extended and applied to the four-port structures presented here. The accuracy of the deembedding techniques was verified by full-wave electromagnetic simulation. Based on the deembedded S-parameters, a SPICE-compatible equivalent circuit model of on-chip coupled transmission lines was extracted. Simulation and measurement results agree well over the entire frequency band from 100 MHz up to 50 GHz.