Phonological patterns often have phonetic bases. But whether phonetic substance should be encoded in synchronic phonological grammar is controversial. We aim to test the synchronic relevance of phonetics by investigating native Mandarin speakers' applications of two exceptionless tone sandhi processes to novel words: the contour reduction 213→21/-T (T≠213), which has a clear phonetic motivation, and the perceptually neutralising 213→35/≠213, whose phonetic motivation is less clear. In two experiments, Mandarin subjects were asked to produce two individual monosyllables together as two different types of novel disyllabic words. Results show that speakers apply the 213→21 sandhi with greater accuracy than the 213→35 sandhi in novel words, indicating a synchronic bias against the phonetically less motivated pattern. We also show that lexical frequency is relevant to the application of the sandhis to novel words, but cannot account alone for the low sandhi accuracy of 213→35.