In this paper we present a comparative study of the optical DC and transient properties of the Si- and GaAs-metal-semiconductor-metal photodetectors (MSM-PD's). The interdigitated metal fingers for the detectors were fabricated on n-type Si and semi-insulating GaAs substrates by using optical lithography and lift-off technique. The width and spacing of the interdigitated metal fingers varied between 1 and 2 μm, and the detector area varied between 100 × 100 μm2 and 500 × 500 μm2. For Si-MSM-PD's we used 150 nm thick aluminum and for GaAs-MSM-PD's 280 nm thick Ti/Pt/Au multilayer metallization for the Schottky barrier formation. The Schottky diode capacitance vs. bias voltage as well as dark and illuminated I-V characteristics as a function of optical power have been measured. Also the photoresponsivity vs. wavelength has been measured for various bias voltages. The speed of the detectors was studied by using a colliding pulse mode-locked laser. The experimental results show that the leakage currents in GaAs-MSM detectors are much smaller than in Si detectors, but still a reasonable photoresponsivity can be achieved also in Si detectors. The speed of the Si detectors could be improved by fabricating them on an epitaxial n+ - n− -structure, which results in a fast impulse response 625 ps (FWHM), 292 ps (fall time).