The indirect determination of water content in lichen thalli by means of impedance measurement is described. This technique applies an alternating current between small clips placed at the margins of lichen thalli. Two zones of sensitivity were apparent in measurements with the foliose lichen Xanthoparmelia chlorochroa. Ready discrimination was provided between fully saturated thalli and thalli with standing surface water films. Similarly, this technique proved highly accurate as a measurement of thallus moisture content at water potentials of –2·5 MPa and less (between 0 and 40% water content by weight in X. chlorochroa). Resolution was lower at intermediate water contents (c. 60–100Δ water content by weight), although general trends were still clearly evident. This technique is ideally suited for unattended measurements of water potential in lichens using currently available data–logger technology and provides high resolution within the range of water potentials crucial to the control of physiological activity in lichens