Some fragments of the Dead Sea Scroll manuscripts were contaminated with castor oil in the late 1950s. We have conducted experiments in order to establish if the AAA pretreatment cleaning procedures conducted on Dead Sea Scroll manuscript samples in the last two dating series (Bonani et al. 1992; Jull et al. 1995) were effective in removing oil contamination. Our experiments show that not all oil contamination can be expected to have been removed by the acid-alkaline-acid (AAA) pretreatment, and that the radiocarbon ages previously reported therefore cannot be guaranteed to be correct. Any samples contaminated with castor oil were most likely reported with ages that are too young by an unknown amount.