This article reports on the results of action research conducted in a university ESP classroom in Oman. The impetus for this research was the practitioner's dissatisfaction with the current practice of introducing the grammatical concept of the English passive and its subsequent results. Framed within the sociocultural theory of cognitive development, this paper investigates the effectiveness of concept-based instruction (CBI). As a pedagogical approach, CBI targets a learner's internalization of the concept of a "language constituent" that assists the learner with the meaning making abilities of sentences where the English passive is used. Twenty-two university students enrolled in an ESP course participated in the study. The data was collected through the teacher's observations, students' artifacts, and students' feedback on the effectiveness of CBI. Data analysis reveals the effectiveness of CBI in heightening learner awareness of the concept of a language constituent, developing learner knowledge of the English passive, and improving their meaning-making abilities at the phrasal and sentential levels.