In all attested periods of the Ancient Thessalian dialect there are two mid-long vowels, written as <E> and <O> in the archaic epichoric alphabet and <EI> and <OY> in the Ionic alphabet. Traditionally the Ionic alphabet spellings have been explained as raised mid-high vowels /eː/ and /oː/. This paper problematizes this assumption, examining evidence from inscriptions transitional between the two scripts, and proposes that the spellings <ΕΙ> and <ΟΥ> may be plausibly interpreted instead as a specifically Thessalian standardized orthography for central-mid vowels /eː̞ / and /o̞ː/.