Using synthetic standards and a combination of GC-MS, direct deposition GC-FTIR and molecular sieving techniques 13 of the 14 possible isomeric ethylmethylnaphthalenes (EMNs) have been identified in crude oils. the remaining isomer, with substituents at ring positions 1 and 8, is apparently unstable under the geological conditions of crude oil formation. EMNs have been identified in crude oils derived from marine and lacustrine source rocks with ages ranging from Proterozoic to Tertiary. the ubiquitous occurrence of the EMNs in crude oils suggests that they are probably derived from microbial sources that have been widespread throughout geological time.