Abstract: The plural of Brazilian Portuguese [w]-final nouns includes an alternation with [j], but the change is partially blocked in monosyllables and following a tense vowel (Becker et al. 2017). In this paper, we present a nonce word study with 115 children ages 7–13 and 43 adults, all participants from the state of São Paulo, showing that blocking in monosyllables is acquired earlier than blocking by tense vowels. We claim that sensitivity to monosyllabicity and vowel tenseness are both due to universal phonological pressures, but the effect of vowel tenseness is learned more slowly because it is limited to the plural morphology in this language. Our results from nonce words are convergent with evidence from innovative plurals and loanword adaptation, showing the primacy of phonological factors over history, orthography, and lexical frequency when it comes to alternations and their acquisition Ver menos