The effects of primary porosity on fluid flow during contact metamorphism were studied in basalts from central East Greenland. The gabbroic Skaergaard magma intruded interbedded massive and aa basalts with mean macroscopic primary porosities of 4% and 11%, respectively. Heat transport from the cooling gabbros led to three metamorphic mineral zones within 1 km of the contact: the actinolite + chlorite zone beyond 250 m, where the mineral assemblage records peak temperatures (7) of ≤550 °C; the pyroxene zone ( T = 700–850 °C); and the olivine zone, within 10 m ( T > 850 °C). In the actinolite + chlorite zone, aa clasts record more extensive mineralogic alteration of igneous minerals than do massive samples. Extents of prograde recrystallization in the olivine and pyroxene zones are 100% in both flow morphologies, but modal volumes of retrograde minerals in the pyroxene and olivine zones are higher in aa units. Extents of prograde reactions do not correlate with primary porosity because they were solid-solid reactions that occurred at high temperatures, whereas retrograde alteration involved low-temperature hydration reactions in which the availability of H 2 O as a reactant, as controlled by porosity, probably influenced reaction extent. In the pyroxene zone, where mineralogic and textural evidence suggests oxygen isotope exchange equilibrium, whole-rock δ 18 O compositions are 1.7‰ to 3.0‰ and are similar or lower in aa units than in massive units at any given distance from the contact. The isotopic ratios suggest average time-integrated fluid fluxes of 3.6 and 4.0 × 10 3 mol cm −2 in massive and aa units, respectively, if fluid infiltration occurred during prograde metamorphism. Similar values were computed assuming that part of the isotopic exchange was retrograde. These differences imply that time-averaged matrix permeability was ~10% higher in aa flow breccias.