A method of measuring the properties of micron-sized slow-wave coplanar transmission lines is presented. The transmission line's propagation constant γ and characteristic impedance Zoare determined directly from the measured S-parameters of a section of transmission line. Measurements are made with microwave "on-wafer" probes. Test data from 0.5 to 40 GHz is presented on a relatively low-loss slow-wave coplanar transmission line fabricated on silicon. The measured quality factor of the transmission line is seen to approach 10 at 11 GHz with a wavelength-compression factor of approximately 5.6.