It was found that the less intense ion peaks of the mass spectra of lower terpenes can play an important role in distinguishing compounds that have similar mass spectra. Two computer programs were developed that generate, from combined gas chromatographic—mass spectra data, F-1 weights (ANOVA) of mass ion peaks between 41 and 300. The library program FIXLIB contains mass spectral and retention data from known mono- and sesquiterpenes, as well as some phenyl propanoid ethers, which are then used for identification of unknowns. The identification program GETID allows rapid comparison of mass spectra and retentions of compounds that fall within specified retention windows. Weighted similarity ratios are calculated which establish the relative similarities of an unknown with the reference compounds falling into a given retention window. An absolute similarity is then calculated for the unknown and that reference compound which has the closest fit, which provides the measure of confidence for the identification. The method was applied to the identification of lower terpenes and phenylpropanoid ethers found in the volatile leaf oils of three true fir and three juniper species, as well as two Douglas-fir varieties. All compounds of previously known identity were correctly identified by the GETID program.