This study aimed at investigating effects of Problem-Based Learning (PBL) on pre-service teachers' performance on conceptual and quantitative problems about concepts of gases. The subjects of this study were 78 second year undergraduates from two different classes enrolled to General Chemistry course in the Department of Primary Mathematics Education a State University in Turkey. Nonequivalent pretest-posttest control group design was used. One of the classes was randomly chosen as experimental group (40), took PBL instruction, and the other was control (38) group, took traditional instruction. Students' achievement of conceptual and quantitative problems in chemistry was measured by Conceptual Problems Gases Test (CPGT) and Quantitative Problems Gases Test (QPGT) as pre and post-tests. The analysis of results showed that students in experimental group had better performance on conceptual problems while there was no difference in students' performances of quantitative problems. The results of the study are discussed in terms of the effects of PBL on students' conceptual learning. (Contains 4 tables.)