Quantifying the share of destructive and nondestructive processes to natural attenuation (NA) of groundwater pollution plumes is of high importance to the evaluation and acceptance of NA as remediation strategy. Dilution as consequence of hydrodynamic dispersion may contribute considerably to NA, however, without reducing the mass of pollution. Unfortunately, tracers to quantify dilution are usually lacking. Degradation though of low-molecular-weight organic chemicals such as BTEX, chlorinated ethenes, and MTBE is uniquely associated with increases in isotope ratios for steady-state plumes. Compound-specific isotope analysis (CSIA) data are commonly interpreted by means of the Rayleigh equation, originally developed for closed systems, to calculate the extent of degradation under open system field conditions. For that reason, the validity of this approach has been questioned. The Rayleigh equation was accordingly modified to account for dilution, and showed that dilution contributed several to many times more to NA than biodegradation at a groundwater benzene plume. Derived equations also (i) underlined that field-derived isotopic enrichment factors underestimate actual values operative as a consequence of dilution, and (ii) provided a check on the lower limit of isotopic fractionation, thereby resulting in more reliable predictions on the extent of degradation.