The etymology of modern fr. Rahin (the name of a river in the Vosges comtoises region) has not been established. As Antoine Thomas had suspected, we are faced here with an oblique case (< -áne), namely *Rohen, whose first vowel opened in aperture to [a]. The simple form Ro(h)e (subject case) was preserved in the Middle Ages through the dehydronymic name of a riverside locale, a name which subsequently changed into Roye. The first designation of the Rahin river as such, *Roe, has its origins in the substantivisation of the lat. rauca f. adj. ‘uttering or making a harsh sound, raucous (of running water)’. The river’s name has since gone through several other transformations. Its oldest form, Rahan(t) (1572), provides us with a new clue into the ancient affinity between the north-eastern part of Franche-Comté with the Franco-Provençal region. This clue seems to be corroborated by two other forms crystallized in the toponymy of Ronchamp.