This study addresses if preservice English as a Foreign Language (EFL) teachers' beliefs about the effectiveness of authentic, commercial, and teacher-made instructional materials can be changed after a semester-long pedagogical treatment. Data were collected from 90 preservice EFL teachers (Experimental: 45, Control: 45) at a public university in Turkey using questionnaires, semi-structured interviews, and retrospective reflection essays. Data analyses involved "t"-tests, multivariate analysis of gain scores, and interview transcriptions. Findings suggested that although change in preservice teachers' beliefs after a well-structured treatment was not very common, they were far from rare. Qualitative results also revealed that some beliefs of participants became more favorable about the effectiveness of some aspects of these materials while others remained unchanged or became less favorable. (Contains 1 figure and 2 tables.)