A collaborative study of the aging of virgin Fe-Ni-C martensites has combined the techniques of transmission electron microscopy (TEM), atom-probe field-ion microscopy (APFIM), and electrical resistometry. Aging at room temperature leads to the rapid development of a finescale structural modulation along 〈203 〉 lattice directions. Atom-probe analysis of Fe-15Ni-lC martensite reveals the formation of carbon-rich regions whose carbon concentration increases with time and approaches 11 at. pct C on prolonged aging. The early stage kinetics of this process are composition-dependent and are consistent with carbon-diffusion control. The morphological features of the aging reaction are explained by elastic strain-energy considerations. In accordance with previous thermodynamic models, it is concluded that virgin Fe-C martensites are unstable and that phase separation occurs by a spinodal mechanism. The martensitic substructure does not appear to exert any substantial influence on this decomposition behavior.