Plagioclase feldspar is often surprisingly porous on a small scale. Pores range in size from less than 1 μm to about 40 μm, and occupy up to 2.3 % by volume. In general, the highest plagioclase porosities occur in rocks which would have had a comparatively “wet” history: normal granites and pegmatites. Rocks which were drier—gabbro, a diabase and an exceptionally dry granite—have much lower plagioclase porosities, as do two lowgrade metamorphic samples. It is suggested that the nearly equant pores represent sites of former fluid inclusions, while the planar cavities of cracklike cross-section may be partially annealed fractures formed early in the history of the rock.