The analysis of essential oils in several species of the Euphorbiaceae family was conducted as part of an intensive program of chemical investigation of odoriferous flora of northeastern Brazil. From 24 different Croton species studied, we have refous nora of northeastern Brazil. From 24 different Croton species studied, we haveselected nine to report on in this work: C. sonderianus, C. essequiboensis, C. argyrophylloides, C. affinis macronifolius, C. rhammi were used for computation of chromatographic retention times (Kovat's indexes) and simplified mass spectra (ms). Confirmations were made by comparison of full ms or other spectra (ms). Confirmations were made by comparison of full Library search programs using mass spectral data and Kovat's indexes were found to be an efficient method for identification of chemical constituents in essential oil analysis.Before 1975 very little information was available about odoriferous flora of northeastern Brazil. At the present time over two hundred essential oils from different plants are under pharmacological and chemical screening as well as botanical investigation.In the Euphorbiaceae family, the genus Croton, due to high incidence and species dispersion, contains by far the largest number of essential oil producing plants in the Brazilian northeast.This communication describes library search programs based on gas chromatography (gc) and mass spectrometric data developed for essential oil analysis and its application to the investigation of ten samples extracted from Brazilian Euphorbiaceae genus Croton; C. argyrophylloides Muell, Arg. (1); C. essequiboensis Kl; C. jacobinensis Baill; C. micans Muell. Arg.; C. sonderianus Muell. Arg. (2); C. nepetaefolius Baill. (3); C. rhamnifolius H.B.K.; C. zehntneri var anethole Pax et $(4)$ Hoff.; and C. zehntneri var eugenol Pax et Hoff.Among several works $(5-11)$ dealing with retention index coupled to mass spectral data in library search programs, only one is concerned with identifications of lower terpenoids and sesquiterpenoids and uses n-heptyl acetate $(=1.000)$ as internal standard to calculate retention times (10).After creating a library of 200 entries and developing a library search program based on mass spectral data for identification of components in essential oils (12), we found it necessary to improve the program with the inclusion of retention indexes.To minimize the standard deviation arising from employing a sole substance as internal standard, we decided to employ a retention index, namely, Kovat's Index $(13)$ obtained by use of a mixture of the essential oil with eight n-alkanes $(C_8, C_9, C_{10}, C_{11}, C_{12}, C_{14}, C_{16}$ and $C_{18}$ as internal standards. This approach greatly improved the identification, especially for those compounds with very similar fragmentation patterns.