編於十三世紀的《蒙古字韻》是研究元代語音的最重要的材料之一。由於此書用八思巴字標音，《蒙古字韻》不但提供了音系上的內容，更提供了難得的語音音值。在《蒙古字韻》中，中古三等韻和四等韻的區別依然在先韻，蕭韻和覃韻中保存。在《蒙古字韻》中，中古三等韻和四等韻的區別依然在先韻，蕭韻和覃韻中保存，並一致地用八思巴字母□和□標示。 然而，由於八思巴學者對於這兩個字母的音值歷來爭執不定，《蒙古字韻》中三等和四等的音值也因此難以確定。試圖解決《蒙古字韻》中三，四等韻的音值問題，本文研究了諸種與之有關的材料。這些材料包括《蒙古字韻》和蒙古文中八思巴字的拼寫規則，以及藏語方言的和漢語吳語方言的材料。研究的結果顯示，八思巴字母□代表的元音要比字母□所代表的要高。字母□實際是個元音前化的標記，其本身並不代表任何一個確定的元音。 根據這兩個字母的音值，在《蒙古字韻》所代表的音系中，四等韻的主元音[ε]低於相對應的三等韻的[e]。這一結論恰和現代吳語中區分三，四等韻的方言中所呈現的區別相符合。

　　　　　The Menggu Ziyun (MGZY) of the 13th century is one of the most important sources for the reconstruction of Yuan phonology. Because of the phonetic nature of the hP'age-pa spellings, this rhyme book provides not only categorical information, but also the invaluable phonetic values of the phonological system. In the MGZY the contrast between MC divisions III and IV is preserved. In rhymes Xian, Xiao and Tan, the valuable phonetic characteristics of these two divisions are consistently represented by two hP'ags-pa letters □ and □. However, because of the phonetic values of these two letters have long been controversial among the hP'ages-pa scholars, the phonetic distinction between divisions III and IV in the MGZY cannot be firmly established. In order to determine the phonetic values of divisions III and IV in the MGZY, this article draws evidence from a variety of relevant materials, which include the MGZY, the Mongolian documents written in the hP'ags-pa script, the Tibetan dialects, and the Wu dialects of Chinese. The result shows the hP'ags-pa letter □ must represent a higher vowel than letter □. Letter □ actually is an umlaut sign. Itself does not represent any certain vowel. Based on the phonetic values of these two letters, it can be established that in the phonological system represented by the MGZY, division IV had a lower main vowel [ε] than division III, and the main vowel of division III could be a relatively high mid vowel [e]. This conclusion is in agreement with the evidence from some of the modern Wu dialects, which still keep the distinction of MC divisions III and IV.