This dissertation empirically investigates the evolution of the gerund and the present participle in Late Latin, and examines on the basis of this investigation the impact of their morphological merging in Old French on their categorial distinction. The results of this thesis confirm the standard hypothesis of the evolution of the gerund and the present participle in Late Latin, but only partly: they support a specialisation of the gerund in adverbial external syntax, but not of the present participle in adjectival external syntax. Thus, the gerund shows signs of ‘converbalisation’, while the present participle does not undergo a process of ‘participialisation’. Like the gerund, the present participle in Late Latin has mostly adverbial external syntax, and is as such more converb-like than participle-like (cf. Haspelmath 1995: 4). This functional similarity between the gerund and the present participle in Late Latin makes that their morphological merging in Old French raises a major problem for their categorial distinction. A considerable proportion of all Old French -ant forms (38%) is categorially indeterminate, that is, not categorisable as either gerunds or present participles – even under the assumption that the gerund and the present participle do not encroach on each other’s distribution, neither in the transition from Late Latin to Old French, nor during the Old French period. This finding leads to the conclusion that the morphological merging of the gerund and the present participle in Old French causes these two types of non-finite verb forms to merge also on a categorial level. The label proposed for this categorial blend is the purely formal one ‘-ant form’.