Dutch language users often insert an inflectional schwa after an adverb, in certain grammatical constructions. The main hypothesis here is that this insertion, which is often ungrammatical, is driven by speakers’ tendency towards regular speech rhythm, which overrides the fine grammatical nuances conveyed by absence of inflection. This rhythmicity hypothesis was investigated in a huge text corpus, viz. all web pages written in Dutch. The proportion of weak-syllable insertion was obtained for a sample of test phrases, varying in rhythmic context around the insertion point. Logistic regression of these proportions shows large and significant effects of rhythmic context on the odds of weak-syllable insertion. Hence, this insertion may well be due to rhythmical factors in language production, in addition to lexical-grammatical factors.