This paper deals with morphological productivity in diachrony, in particular it addresses the issue of the quantitative evaluation of productivity within a given time span. Adopting Baayen’s (1992; 2001; 2008) corpus-based quantitative approach which considers productivity as the probability of encountering a new type when sampling a large corpus, the paper shows the evolution of two competing suffixes -mento/-zione in Old Italian from the 13th to the 16th Centuries. On the basis of four separate corpora drawn from LIZ 4.0 (Letteratura Italiana Zanichelli), it is demonstrated how the productivity of the suffix -mento, within the time span of four centuries, remains constant, while the suffix -zione displays diachronic variability. Apart from diachronic considerations regarding this situation, the paper also highlights some technical aspects, such as the use of LNRE models (implemented in the package zipfR, a tool for lexical statistics in R, cf. Baroni — Evert, 2006; Evert — Baroni, 2007; Baayen, 2008), as well as some well-known limitations and constraints inherent in quantitative analyses of diachronic corpora.