Architecturally appropriate and technically effective conservation and maintenance of historic masonry requires meaningful characterization of existing materials. Sampling from important historic monuments is often not allowed, so there is a need for non-destructive in-situ test. Portable probe gas permeability, developed for use in petroleum reservoir studies, has the clear potential for application in practical conservation. In particular the application of gas permeametry in-situ may help in the study of weathering and decay of historic masonry materials. A pilot experiment was carried out to evaluate the applicability of steady state portable probe permeametry for the in-situ characterization of historic natural sandstone masonry. The in-situ surface gas permeability of recently cleaned and original un-cleaned ashlar sandstone masonry was compared and the repeatability and variability of the measurements was evaluated. The method was found to be capable of assessing the permeability contrast of the cleaned and un-cleaned sandstone, though some limitations of the test method were identified.