Photosynthesis imparts a unique isotopic signature to the O2 it produces via “isotopic clumping,” a phenomenon in which molecules accumulate two or more heavy isotopes. Molecules with clumped isotopes are uncommon, but modern mass spectrometers can reliably measure their abundances. Isotope clumping as a field is only 10 years old and has been used to deduce the formation temperatures of minerals. When a system is at chemical equilibrium, isotopic clumps are found more often than random chance would predict. But on the basis of a study of isotopically clumped O2 (18O18O and 18O17O) in hyacinth plants, Laurence Yeung, Jeanine Ash, and Edward D. Young of UCLA show that photosynthesis actually produces isotopically clumped O2 less often than chance would predict (Science 2015, DOI: 10.1126/science.aaa6284). In another investigation, a team lead by David T. Wang and Shuhei Ono of MIT studied isotopically clumped methane, 13CH3D. They showed that its relative abundances can be used to identify methane sources produced by cattle (Science 2015, DOI: 10.1126/science.aaa4326). “Clumped isotope anomalies will help place much-needed constraints on biogeochemical sources, sinks, and budgets of O2 and CH4,” writes Johns Hopkins University’s Benjamin H. Passey, in an accompanying commentary.