On the basis of an acoustic phonetic analysis of monophthongs and diphthongs, this paper describes vowel phonology in the Taiyuan Jin dialect. The results show that Taiyuan has a comparable but different vowel inventory for C(G)V versus C(G)VN syllables. And the vowel contrast is dramatically reduced in checked syllables. The asymmetry between falling and rising diphthongs suggests a dynamic account of vowels, rather than a sequential taxonomy of vowels into monophthongs and diphthongs. Phonetically, monophthongs are composed of a static spectral target, falling diphthongs are composed of a dynamic spectral target, and rising diphthongs are sequences of two spectral targets. Phonologically, falling diphthongs are grouped with monophthongs, rather than rising diphthongs.