本文強調在英語教授專業過程中，英語非為母語的專業教師所持有之情境脈絡信念，所扮演的重要中介角色。透過探討兩所大學中四位社會科學學系教師英語授課經驗與看法之質化個案研究，本文將探討前述教師的情境脈絡信念所發揮的三種功能：(a)建立英語授課之正當性、(b)批評學校英語授課政策、與(c)設計適性英語授課課程。有別於過去教師信念對英語授課的負面描繪，本次研究發現情境脈絡信念產生透視鏡功用，使教師藉此提升自我效能，決定是否開始、持續、與建議他人以英語授課，證明他們英語授課教學方法，並瞭解學生語言需求/權利。文末也將對大學英語授課教師之教學發展提出建議。

This paper argues that nonnative English-speaking (NNES) content instructors’ contextual beliefs function as critical mediators in teaching discipline-specific content in English. Drawing on data from a qualitative case study investigating four social science instructors’ perceptions and experiences in two universities in Taiwan, the paper discusses the functions of these instructors’ contextual beliefs in three principal ways: (a) establishing legitimacy of adopting English as a medium of instruction, (b) critiquing school policies, and (c) designing adaptive content courses in English. In contrast to the negative influence of teacher beliefs on EMI, contextual beliefs as reported herein emerge as a lens through which instructors’ self-efficacy is increased, their decisions to begin, remain, and recommend adopting EMI are made, teachingmethods are justified, and students’ language needs/rights recognized. This paper concludes by discussing the implications of the study for university instructors’ professional development.