本文運用結構分析以及方言比較等方法，探討閩東方言一項較少被討論的-u-、-y-混同音變，將之分為兩類進行觀察比較：一是介音-u-與-y-在聲母為舌齒音、主要元音為-o-的條件下逐漸混同合流，此音變普遍發生在閩東各地，但各地結構限制與變化方向不一；二是主要元音-u-與-y-在特定韻尾條件下發生同化音變，舌根韻尾條件下-y-後化為-u-，舌尖韻尾或前高元音-i韻尾條件下-u-前化為-y-，此音變僅發生於北片方言點。研究結果發現：閩東方言-u-、-y-混同音變可以運行相反的演變方向，而且各地結構限制不盡相同：介音混同部分，除了以往所提出的必要條件：聲母為舌齒音、主要元音為-o-，少數方言點還涉及韻尾結構的限制；而主要元音混同部分，乃以韻尾作為音變條件，但各地結構擴散情形有所差異。除了從結構條件討論這類音變的運作與變化趨向，本文更進一步從語言系統整合的角度切入，探討層次競爭啟動結構重整的可能性，也就是說-u-、-y-混同音變的深層動因可能來自不同層次變體長期競爭互動，語言系統逐步將之有序化的自然需求，並且促發同類語音結構發生系統性變異，因此不同方言點的結構限制與變化方向遂不盡相同。

The author applied the method of structural analysis and dialectal comparison to study the merging of -u- and -y- in Eastern Min. The mergers could be divided into two groups. One is a merger of medial -u- and -y- on condition that the initials are dental or alveolo-palatal consonants and the main vowel is -o-, which operates in most Eastern Min dialects. The other is a merger of main vowels -u- and -y- by assimilation to the endings, which only operates in the northern area of Eastern Min. The research shows that the merging of -u- and -y- could be carried out in reverse trends and under diverse structural constraints. The author further discusses the probability that competition among strata actuates structural reintegration and systematical sound changes. As a result, the merging of -u- and -y- is implemented through diffusion. And each place may stand on the different stage of the diffusion and walk towards the opposite directions.