Correlations of several extensive ignimbrite units in the 21°30′–23°30′ S portion of the Central Andes of northern Chile have been achieved by field mapping, geochemical ‘fingerprinting’, and 50 new K-Ar ages. The most useful criteria for fingerprinting are microprobe determinations of Fe, Mg, Mn and Ti for biotite, and Fe, Mg and Mn for hornblende, trace-element compositions (Ba, Sr, Y and TiO 2) of the dominant pumice type in each ignimbrite; in some instances non-welded bulk-matrix samples can also be used. Other criteria such as trace-element compositions of glass and modal phenocryst abundances are less effective but may be used in conjunction with other criteria. Using this approach, a very extensive unit, the 8.33-Ma-old Sifon ignimbrite, previously thought to be at least three separate units, has been identified. It occurs over an area of ∼20,000 km 2 and has a volume of about 600 km 3 within northern Chile alone. An equivalent volume is thought to occur in southwestern Bolivia. Conversely, while the 4.18-Ma-old Puripicar and 4.09-Ma-old Atana ignimbrites appear to be parts of the same extensive ignimbrite unit, variations in their mineral assemblage, mineral chemistry and trace-element characteristics indicate separate magmatic and chemical evolution, and they are considered separate units. Correlations of extensive ignimbrites are problematic and should be based on a systematic approach to identify the best criteria for correlation. Our observations in the Central Andes lead us to conclude that such correlations can only satisfactorily be effected using a combination of criteria. Where such correlations are claimed on a less systematic basis, these should be treated with a measure of caution.