Old French (OF) determiners (D), which are optional, show a three-way split between definite (def), indefinite(indf), and expletive(expl) D. We develop a nano-syntactic analysis of these three paradigms, according to which the nominal spine is associated with a series of functional heads that include Number, Gender, D, and Kase. We test the predictions of the formal analysis with a quantitative analysis of corpus data from two 12th century Anglo-Norman texts – Le voyage de saint Brendan (B) and Lais de Marie de France (MdF) – which indicates that over a 60-year span, there are changes in the distribution of D. This presents itself in three ways. First, a decline in expletive D in MdF correlates with an increase in the use of D with masculine (m) non-count nouns (nNON-CT) Second, while B lacks an overt indefinite plural (pl) D, MdF has one in the form of des. Third, with count nouns(nCT), while feminine (f) nouns favour the absence of determiners in B, there is no gender effect in MdF. While the first two changes are predicted by the formal analysis, the third is not. More broadly, the results of our quantitative study provide a more nuanced picture of the factors that govern the distribution of D in OF: they confirm that – relative to conditioning the absence of D (D-drop) – definiteness, grammatical function, and number are stable factors, gender is not a stable factor, and word order does not play a significant role.