八斯巴字字母表示元音時 (元音性 h) 的音值，是一個學者們深感興趣，但至今未有定論的問題，本文對此提出了 新的研究方法。我們認為八斯巴字拼寫的根據是當時元代漢語語音的表層形式，因此不能以其深層形式或現代學者建立的構擬模式作為研究的依據。從此出發，本文由新的角度，特別是語音升學的角度，來解決八斯巴字拼寫上的問題。 本文由四部份組成。在第一部分，我門列出了元音性h的出現範圍，併列舉了各種轉寫方案。在第二部分，我門討論了轉寫中的幾個關鍵問題。我門明確指出八斯巴字創制，是和語和藏語之間的一種交互作用關係。因此，漢語的表層形式以及這些形式又是如何為八斯巴字創始者所感知的，是理解八斯巴字拼寫的一個關鍵。在第三部分，我門提供了確定元音性h的語音功能的新的證據。這些證據主要來自聲學語音學和古藏語。第四部份，我門從語音聲學的角度解釋了為何元音性h在喉擦音聲母後面不出現蔗一特殊的音系現象。我們的解釋解決了〈蒙古字韻〉中一些一等字和三等字一起排列的問題。 文章的主要結論是：元音性h只是一個語音特徵並不代表一個獨立的音段。它只是用來表示它後面的元音後話。八斯巴字 hi 表示中央音[i],h(a) 表示後元音 [a]。在性質上來說，這一用法可以說和八斯巴字母作為一個表示元音前的語音特徵上是相一制的。

The phonetic values of the vocalic h have interested hP’ags-pa scholars for decades but still remain controversial. The new approaches taken in this study are based on our view that the phonetic values of the hP’ags-pa spelling should be interpreted according to the surface values of the Chinese syllables instead of their underlying of reconstructed values. With such a shift in the basic understanding, acoustic phonetics, which is largely ignored by the Yuan phonologists, should play a crucial role in the interpretation of the hP’ags-pa spelling. In the first part, for the purpose of convenience, we show the phonological environments in which the vocalic h appears and summarize various interpretations proposed so far. In the second part, we discuss some critical issues involved in the interpretations. We point out that the invention of the hP’ags-pa script was basically a linguistic interaction between Chinese and Tibertan. Thus we focus on the differenct between the Chinese phonetic forms and forms perceived by the inventors of the hP’ags-pa alphabet. In the third part, from different approaches, we offer new evidence for determining the phonetic value of the vocalic h. In part four, from a phonetic point of view we explain the deletion of the vocalic h after guttural fricatives. We also explain a peculiar phenomenon in the MGZY, which is why some MC division-I syllables appear along with the MC division-III syllables. In this paper we suggest a unified solution for the various usages of the vocalic h. Like letter Ц letter h actually is just a modifying feature on the following vowel. Its phonetic value is [-front], which changes [i] to a central vowel [Ï] and [a] to a back [β].