This paper focuses on the e/i theme vowel class of verbs in Slovenian to bring together two seemingly unrelated debates; (i) the debate on the status of derivational affixes as roots within the framework of Distributed Morphology and (ii) the debate on the correlation between theme vowel classes with certain argument structures in Slavic. Focusing on Slovenian, our core data will come from active l-participles that are used adjectivally as an unaccusativity diagnostic. We take these

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... s to create a list of 109 unaccusative verbs. We show that (i) no unaccusative verbs belong to the two largest theme vowel classes in Slovenian (a/a and i/i), whereas (ii) the two big theme vowel classes tend to get accusative arguments quite frequently. Most importantly, (iii) the e/i-class stands out since more than one half of the unaccusative sample falls into. The e/i-class is furthermore the only theme vowel class whose theme vowel surfaces in adjectival l-participles, the theme vowel class to which inchoatives belong and behaves uniformly with respect to stress. Based on the uniform behavior of the e/i-class which sets it apart from other theme vowel-classes, we will argue that the vowel of this class is better analyzed as a root.