The relative abundance of rare stable isotopes incorporated into biological molecules carries information on used substrates, biosynthetic pathways, environmental conditions and post-depositional alteration processes. Advances in high-resolution MS now allow us to study the internal isotope anatomy of individual molecules. By looking at isotope 'clumping' and determining the likelihood of rare isotope intramolecular patterning, we explore information that can be recorded in the 'isotope anatomy' of molecules.