Isolated and integrated grammar instruction are two approaches to grammar teaching that can be implemented within a form-focused instruction (FFI) framework. In both approaches, instructors primarily concentrate on meaning, and the difference is in the timing of instruction on specific language forms. In previous studies, researchers have observed that the match between teachers' and learners' beliefs related to the effectiveness of instructional approaches is an important component in predicting the success of grammar instruction. In this study, we report on the psychometric properties of a questionnaire designed to measure students' perceptions of isolated and integrated FFI taking place in Iranian secondary schools. The Iranian context is interesting with regard to approaches to grammar instruction in light of recent policy reforms that emphasize isolated FFI. Using a combination of principal components analysis and Rasch measurement theory techniques, we observed that Iranian students distinguish among the two forms of grammar instruction. Looking within each approach, we observed significant differences among individual students as well as differences in the difficulty for students to endorse different instructional activities related to both isolated and integrated instruction. Together, our findings highlight the importance of examining students' beliefs about the effectiveness of approaches to grammar instruction within different instructional contexts. We discuss implications for research and practice.