With the example of the Chao’an dialect, the present paper discusses the significance of the dialect materials collected by the late Proffessor Y. R. Chao during his second field trip in 1928-29. Constituting the first accurate phonetic record of various Southern dialects during the early part of 20th century, these materials serve as landmarks in the periodization of modern phonological changes in Chinese dialectology. They also play the role of checking and affirming the value of earlier “less professional” materials most frequently complied by missionaries, which often form important earliest documents for the various dialects but which have not been fairly evaluated or adequately used ofr the historical study of these dialects. The Chao an materials demonstrate the wealth of information provided form the field: they affirm the distinctions of the rime categories established in the tradiotinal rime dictionary of The Fifteen sounds of Chaozhou, present the last remnant of evidence for an –n ending in the Chaozhou dialect, affirm the value of various previous colloquial materials, reveal the conditions and process of the attrition of consonantal endings in Chao’an, and provide evidence for earlier distinction of the Double rimes in the Xian rime group.