A mutual impedance probe, also called quadrupolar probe or permittivitymeter, measures the complex permittivity of materials with a spatial resolution comparable to the average separation between its four sensors. This instrument is ideally suited for the detection of subsurface water deposits at shallow depths on Mars, since water mixtures are generally characterized by relatively large dielectric constant and conductivity. Permittivitymeters have been developed for commercial and space applications. An instrument identical to that which will land on Titan in 2004 has been tested with success in the field, and the results obtained on humid sand and in dry snow are presented. The possible applications of mutual impedance probes to the localization of water on Mars are discussed.