The cartulary commonly known as Becerro de San Pedro de Arlanza is a twelfth-century codex recording the main documents of the monastery s archive from the early tenth to the mid-twelfth century. The codex was lost during the 1936-39 Spanish Civil War, so its contents are only known thanks to Luciano Serrano's 1925 publication of the Arlanza collection. Exhaustive analysis of the data provided by him sheds some light upon the making of the cartulary and its role in the formation of the Arlanza documentary collection.