We report the successful application of magnetic techniques to the study of rock varnish, a surficial coating that forms on rocks in arid environments. Using thin wafers of varnished and unvarnished greywacke sandstone, we have detected a stable remanent magnetization that can be unambiguously attributed to the rock varnish. We have also measured several rock magnetic properties of the rock varnish and have shown that the primary magnetic carrier is either magnetite or maghemite. Our results demonstrate the feasibility of applying magnetic techniques to the study of geomorphic surfaces. Subsequent rock magnetic and paleomagnetic studies of rock varnish should provide new information of rock varnish and about the validity of a new chronological technique known as rock varnish dating.