Compound-specific isotopic analysis of biomarker molecules yields information about biogeochemical processes in the environment in which the compounds were formed. The utility of this information, as well as the level of confidence in the data interpretation, increases with the number of isotopes investigated. Here we present two contrasting cases to illustrate (i) how uniformity of the data for multiple isotopes measured within a single compound class strengthens the argument for a common source, and (ii) how variability of the isotope data within another compound class can be decoupled algebraically to yield mixing ratios when there are multiple sources. In the first case, the d¹³C, D¹⁴C, and dD values of sterols from Santa Monica Basin surface sediments all indicate that these compounds derive from marine planktonic biomass. In the second case, we show that the isotopic composition of n-alkanes extracted from the same sediments is not uniform. This variability results from mixing three major endmembers, petroleum, modern terrestrial plant waxes, and continental rock weathering. These examples demonstrate the importance of a multivariable approach to organic geochemical studies.