Photosynthesis generates similar to 100 gigatons of carbon per year and drives many terrestrial geochemical cycles. Magnesium is the metal center in chlorophyll and is thus central to photosynthesis. Here we report measurements of the Mg isotopic composition of chlorophylls in English Ivy (Hedera helix L.) showing that chlorophyll-a is isotopically heavier than chlorophyll-b (Delta Mg-26(Chla-Chlb) = 0.434 % (+/- 0.148) and Delta Mg-25(Chla-Chlb) = 0.241 % (+/- 0.108)). The measurements are consistent with electronic structure calculations of the isotopic fractionation, implying that chlorophyll-a and -b are in isotopic equilibrium. Mg isotopic equilibrium suggests that the metal is labile in these molecules during the chlorophyll cycle after the various forms of chlorophyll are biosynthesized.