The problem here is to determine whether the OLF i-umlaut reflexes of PGmc. */u ū ō aw/ were phonemically distinct from their non-umlaut reflexes. This is interesting because: (1) the OLF orthography offers no consistent evidence for phonemic contrast between umlaut reflexes of these sounds and their non-umlaut counterparts; (2) Middle Low Franconian (Middle Limburgic) does show phonemic contrasts here; yet (3) there was apparently no factor present in Old Low Franconian that would have led to the phonemic splits implied by the Middle Limburgic contrasts. The present paper makes the assumption that a consistently applied orthographic distinction implies a phonemic contrast, and points out some obvious limitations of this assumption. The vowels of weakly stressed final syllables are discussed, and it is suggested that previously distinct vowels had merged in a single vowel in this position. It is finally concluded that the OLF reflexes of PGmc. */u ū ō aw/ in umlaut position were phonemically distinct from the reflexes of those sounds in other positions.