The occurrence of the archaeal biomarkers crocetane and pentamethylicosane (PMI) in sediments and crude oils was investigated using GC-MS and GC-MS-MS. Selected ion monitoring (SIM) and multiple reaction monitoring (MRM) protocols designed to detect the C-20 and C-25 irregular isoprenoids were evaluated. A method for the preferential detection of crocetane over phytane, its more abundant isomer, was refined using authentic compounds. Crocetane was found to be present in modern sediments from both aerobic and anaerobic water columns and in sediment from the 1640 Ma Barney Creek Formation, providing evidence for the antiquity of its biosynthesis. Crocetane was also detected in several Perth Basin crude oils and in the Kockatea. shale, their putative source rock, and in Canning Basin oils, although MRM analysis could only verify its occurrence in Canning oils of Devonian age. Equivocal data from Ordovician and Carboniferous Canning Basin oils highlight the need for extreme caution when attempting to distinguish between acyclic isoprenoid isomers. PMI was identified by MRM in a Miocene Monterey sediment and in a modern Antarctica sediment. On the other hand, the C-25 isoprenoid hydrocarbon peak detected in Triassic and older sediments was consistently identified as the regular isomer (I-25 reg) whenever it was sufficiently abundant for MRM to be applied. Crown Copyright (C) 2003 Published by Elsevier Ltd. All rights reserved.