Transient effects of optically modulated RF-switches, measured with a large-signal vector network analyzer (LSNA) are discussed in this paper. Light is used to influence the characteristics of a coplanar waveguide (CPW) designed in a multilayer thin-film multichip module (MCM-D) technology on high resistivity Silicon (HR-Si). Under static illumination of the CPW line, measurements demonstrate an insertion loss contrast of more than 35 dB above 20 GHz. With a modulated illumination input in the kHz-range, the study of the insertion and return loss show transient effects depending on the carrier frequency and substrate parameters. Time dependent S-parameters are used to study the switch behavior of the CPW line under illumination. The static physical model, used in 3D-electromagnetic (EM) simulations, is used to explain the time dependent behavior.