AbstractMorphological concatenation often triggers phonological processes. For instance, addition of the plural suffix /-ən/ to Dutch nouns causes vowel lengthening in some nouns due to the stress-to-weight principle ([xɑt] vs. [ˈxaː.tən] ‘hole’). These kinds of processes often apply only to a subset of words – not all Dutch nouns undergo this process ([kɑt] vs. [ˈkɑ.tən] ‘cat’). Nouns need to be lexically indexed as either undergoing this process or not. I investigate how phonological grammar and lexical indexation are learned when learners are confronted with data like these. Based on learnability considerations, I hypothesise that learners acquire a grammar with default non-alternation, so that novel items are treated as non-alternating. I report the results of artificial language learning experiments compatible with this hypothesis, and model these results in a version of the Biased Constraint Demotion algorithm (Prince & Tesar 2004).