Information storage at the molecular level commonly entails encoding in the form of ordered sequences of different monomers and subsequent fragmentation and tandem mass spectrometry analysis to read this information. Recent approaches also include the use of mixtures of distinct molecules noncovalently bonded to one another. Here, we present an alternate isotope ratio encoding approach utilizing deuterium-labeled monomers to produce hundreds of oligomers endowed with unique isotope distribution patterns. Mass spectrometric recognition of these patterns then allowed us to directly readout encoded information with high fidelity. Specifically, we show that all 256 tetramers composed of four different monomers of identical constitution can be distinguished by their mass fingerprint using mono-, di-, tri-, and tetradeuterated building blocks. The method is robust to experimental errors and does not require the most sophisticated mass spectrometry instrumentation. Such isotope ratio-encoded oligomers may serve as tags that carry information, but the method mainly opens up the capability to write information, for example, about molecular identity, directly into a pure compound via its isotopologue distribution obviating the need for additional tagging and avoiding the use of mixtures of different molecules.Read less <