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This paper presents experimental results for line-to-line coupling between adjacent coplanar waveguide (CPW) transmission lines. The CPW lines are fabricated on glass and low resistivity CMOS grade Si substrates coated with a 20μm cyclotene polymer. From the measurement results it is found that the coupling increases with frequency on low resistivity silicon substrate while the coupling on glass shows relatively constant peaks. A spacing of 600μm between CPW lines is required to avoid parasitic coupling less than 30dB for frequencies up to 30GHz when fabricated on low resistivity silicon, while a spacing of 200μm is required when fabricated on glass. The conductor width (W) and the slot width (S) for low resistivity silicon is 80μm and 45μm, and on the glass substrate W = 150μm, S = 30μm. Based on full-wave EM simulation results, it is argued that the primary coupling mechanism is through the interaction of higher-order modes.