Most of the territory of modern-day Finland was part of the Swedish kingdom during the medieval period. Around 1350 Swedish replaced Latin as official language while Finnish essentially remained a language for oral communication until the 16th century. Nevertheless, traces of Finnish, mostly proper names, can be found in various kinds of Old Swedish charters. Occasionally scribes even rendered Finnish toponyms in locative case forms, in particular when indicating designations of origin for named individuals. This surprising occurrence of Finnish case-endings has generally been considered a result of the deficient Finnish language proficiency of the Swedish scribes. In this paper, it is shown that, contrary to earlier views, the use of Finnish in Old Swedish charters follows a clear pattern, which suggests that scribes in Finnish-speaking areas of the Swedish realm were at the very least able to understand some Finnish, and used Finnish in a conscious manner when suitable.