The Proto-Creole of the Gulf of Guinea is the common ancestor of the Santome, Angolar, Lung’Ie and Fa d’Ambô languages. Based on a supposed occurrence of vowel harmony (vh) in these modern languages and the proven influence of the languages of the Niger Delta (HAGEMEIJER, 2009), in which vh is attested, this study aims to discuss the existence of vh processes with [atr] mid-vowels in the Proto-Creole of the Gulf of Guinea, considering as a starting point its phonological and lexical reconstruction (BANDEIRA, 2017). Therefore, we intend to verify what phonological parameters related to vh in the literature (trigger, target, domain, application direction and blocking element) indicate about vh processes in Proto-Creole, based on the lexical reflexes of a set of cognates in daughter languages and proto-forms. We will show that many of the items interpreted as harmonic derive from the maintenance of mid-vowels in Portuguese etyma and from the insertion of a vowel copy in order to avoid roots ending in consonants in the Proto-Creole of the Gulf of Guinea. In this analysis, we will argue for the hypothesis according to which parasitic vh processes (COLE; TRIGO, 1988) with [atr] mid-vowels in contiguous syllables occurred in Proto-Creole within the prosodic word domain. However, the vh was limited because, based on the observation of proto-forms and harmonisation patterns with [atr] mid-vowels in contiguous syllables, there were many cases in which there was neutralisation of final non-stressed mid-vowels and, therefore, no application of the harmony process in favouring contexts.