The aim of this study was to reveal the effects of Problem-based Learning (PBL) on the metacognitive awareness and attitudes toward chemistry of teacher candidates with different academic backgrounds. The study was carried out on one group using both pre-and post-test experimental studies. The findings of the study were obtained through quantitative approaches. The sample of the study was 70 "first-year undergraduate students" at a state university in Turkey taking General Chemistry/General Chemistry-II classes. The study was implemented during the spring semester of the 2011-2012 academic years and for a period of 20 hours. Quantitative data was obtained using the Metacognitive Awareness Inventory and the Chemistry Attitude Scale. Two dependent sample t-tests were used for the pre-and post-test comparisons. The findings showed that PBL was more effective in developing metacognitive awareness levels of students with weak science background knowledge compared to those with strong science backgrounds. In addition, the findings showed that PBL was effective in increasing the attitudes positively toward chemistry of students with weak scientific backgrounds. (Contains 3 tables.)