English speakers disfavor compounds containing regular plurals compared to irregular ones. Haskell, MacDonald and Seidenberg (2003) attribute this phenomenon to the rarity of compounds containing words with the phonological properties of regular plurals. Five experiments test this proposal. Experiment 1 demonstrated that novel regular plurals (e.g., loonks-eater) are disliked in compounds compared to irregular plurals with illicit (hence less frequent) phonological patterns (e.g., leevk-eater, plural of loovk). Experiments 2–3 found that people show no dispreference for compounds containing nouns that merely sound like regular plurals (e.g., hose-installer vs. pipe-installer). Experiments 4–5 showed a robust effect of morphological regularity when phonological familiarity was controlled: Compounds containing regular plural nonwords (e.g., gleeks-hunter, plural of gleek) were disfavored relative to irregular, phonologically-identical, plurals (e.g., breex-container, plural of broox). The dispreference for regular plurals inside compounds thus hinges on the morphological distinction between irregular and regular forms and it is irreducible to phonological familiarity.