This study analyzes references cited by articles published in ten American Chemical Society journals between 2011 and 2015. The median age of references was 6 years. On average, 44 percent of the references were five years old or younger, and only 11 percent were more than 20 years old. There appears to be a modest increase in references to older sources, possibly due to the increased availability of older articles online. References tended to be concentrated on a small core of journals. Overall, 20 percent of the journals cited accounted for 80 percent of the references. However, there was considerable variation among sub-disciplines