Microwave attenuation of coplanar waveguides fabricated on high–resistivity silicon substrates with SiO2insulation layer is investigated from 10 GHz up to 600 GHz by optoelectronic time–domain measurements. They are performed directly on the wafer employing a freely positionable photoconductive switch for picosecond–electric–pulse injection and an electro–optic crystal for pulse detection. The attenuation is significantly altered over the whole frequency range by free–carrier absorption resulting from inversion and accumulation effects at the Si/SiO2interface. ©1995 American Institute of Physics.