This study investigated the effectiveness of combining conceptual change text and concept mapping strategy on students' understanding of diffusion and osmosis. Students' conceptual understanding of diffusion and osmosis was measured using the Diffusion and Osmosis Diagnostic Test developed by Odom and Barrow (1995). The test was administered as pretest and post-test to a total of 44 ninth-grade students in two intact classes of the same high school located in an urban area. The experimental group was a class of 24 students who received concept mapping and conceptual change text instruction. A class of 20 students comprised the control group who received a traditional instruction. Group Assessment of Logical Thinking Test (GALT) and pretest scores were used as covariates in this study. A pretest-post-test control group design utilising the analysis of covariance (ANCOVA) showed a statistically significant difference between the experimental and control groups in the favour of the experimental group after treatment. The results indicated that while the average percentage of students in the experimental group holding a scientifically correct view had risen from 22.5% to 54.1%, a gain of 31.6%, the percentage of correct responses of the students in the control group had increased from 19.1% to 38.7%, a gain of 19.6% after treatment.