Old books smell like vanilla because the lignin in paper breaks down into vanillin as it ages.

The chemical process behind the 'old book smell' is known as acid hydrolysis. Paper produced between the mid-19th century and the late 20th century contains high amounts of lignin, a complex organic polymer that provides structural support to trees. As this lignin reacts with light, heat, and moisture, it undergoes a slow degradation process that releases various volatile organic compounds (VOCs) into the air.One of the primary aromatic byproducts of this breakdown is vanillin, which is chemically identical to the extract derived from vanilla beans. Other compounds released include benzaldehyde, which adds an almond-like scent, and ethyl benzene, which contributes a sweet odor. Together, these chemicals create the nostalgic bouquet often associated with historic libraries and used bookstores.In 2009, researchers at University College London developed a technique called 'material degradomics' to analyze these scents. Led by Matija Strlic, the study published in the journal Analytical Chemistry explained how these VOCs serve as a diagnostic tool. By 'sniffing' the air around a collection, conservators can determine the chemical stability and degradation state of paper without damaging the artifacts.This method is particularly useful for identifying 'vinegar syndrome' in cellulose acetate films or high acidity in wood-based paper. Because modern paper is often treated to be acid-free and has most lignin removed, newer books will likely never develop this specific vanilla-like aroma. This makes the scent a unique characteristic of historical documents and vintage publications.