Data and code for the article "Substantive bias and variation in the acquisition of vowel harmony". This study investigates substantive bias, a phenomenon wherein learners exhibit a preference for phonetically motivated patterns in language acquisition. The study presents evidence that variable input, rather than categorical input, can activate substantive bias. Native speakers of Hong Kong Cantonese were randomly assigned to categorical or variable training conditions for vowel backness harmony or disharmony, or to a no-training control condition. Results indicate that participants in the categorical and control conditions did not exhibit a bias towards either pattern. However, participants in the variable conditions displayed a bias towards vowel harmony, suggesting that input variability can strengthen the effect of substantive bias. These findings contribute to our comprehension of the role of input variability in phonological learning and the mechanisms involved in the acquisition of phonetically motivated and unmotivated phonological patterns.