Both Old French meters and their Modern French descendants are usually thought to lack the internal binary constituent structure of, say, English or German iambic verse. In this article, however, an underlying iambic structure for the Old French octosyllable is established through quantitative analysis of a large corpus of texts written from c. 975 to 1180 (42 distinct works, including over 22,000 lines). Because no texts conform absolutely to the grammar of English iambic verse (Halle & Keyser, 1971; Kiparsky, 1977), certain measures are proposed for the degree to which a sample deviates from the iambic pattern; these values are then compared with the (chance) deviation of normal Old French prose. A significant correlation emerges between these measures and date of composition, author, and genre: early texts are almost perfectly iambic, and late 12th-century texts approach, but do not reach, chance levels. It is concluded that the grammar of meter used by Old French authors underwent a gradual change during the 12th century, a change comparable to more familiar phonological and syntactic changes.