Despite recent proliferation in scholarship on English research writing (ERW) and its teaching, ERW teachers' developing knowledge remains under-researched. This multiple case study, featuring four ERW teachers working in the same university in Central China, builds on the ped-agogical content knowledge (PCK) framework to delineate the complexities in participants' knowledge-practice nexus. Data sources include observations, interviews, journal entries, and teaching artifacts. Data analyses reveal that the complex interactions between PCK components have resulted in idiosyncrasies in participants' instructional decisions or practices concerning selecting teaching points, utilizing own disciplinary articles, deciding on pedagogical design, and assigning writing tasks. It is also found that participants who are more assured in their PCK repertoire tend to feel more secure in teacher-student interactions and adopt more student-centered pedagogies. Participants' knowledge-practice nexus exhibits complex system features of self-organizing and self-transforming, as well as self-knowing. Implications for transitional ERW teachers' practices and professional development are discussed./journal/ijal 2 ZHANG and GUO K E Y W O R D S complex systems theory, English research writing teachers, instructional practices, pedagogical content knowledge , , , , , , , , "-" , ; ; ; ; 1 INTRODUCTION With English being increasingly established as the international language of scientific communication, the demand for instruction on English research writing (ERW) is being widely felt in tertiary institutions around the world, with universities in Chinese mainland being no exception (Cargill et al., 2018; Li et al., 2020). The inroad of ERW into tertiary-level English teaching programs requires many college English teachers to make a transition from teaching English for general purposes (EGP) to teaching ERW. Despite the recent proliferation in scholarly reports on ERW teaching (for a review, see Li & Flowerdew, 2020), transitional ERW teachers' knowledge as cognitive underpinnings for their instructional behaviors remains under-researched. Developing a nuanced understanding of ERW teachers' instructional practices and knowledge development as complex and reciprocal processes would hold profound implications for prospective or less experienced ERW teachers still "learning the ropes of genre-based pedagogies through practice" (Li et al., 2020, p. 1). Due to their EGP-only educational and career background and the lack of established curricular setting for ERW teaching in many Chinese universities, most transitional teachers rely on their developing knowledge and skills to handle the multiple tasks of "investigating needs and specialist discourse, developing courses and materials as well as classroom teaching" (Basturkmen, 2014, p. 17). This practice-embedded knowledge development entails that transitional ERW teachers' knowledge and instructional practices would "shape and reshape each other within and across complex systems" (Yu et al., 2020, p. 3), rendering their knowledge-practice nexus a good candidate for complex systems theory (CST) informed analysis.