Chemical compatibility tests were conducted on high-density polyethylene geomembrane samples using the Comprehensive Testing System (CTS) under low- and high-displacement conditions. CTS development between the two sets of data (low-and high-displacement) was found to significantly reduce the friction within the testing cell. This friction reduction was apparent from the decrease in delta modulus from the larger values obtained during low-displacement testing to the smaller values obtained during high-displacement testing. The results of the high-displacement testing showed statistically significant differences between delta modulus results at the 95% confidence interval (probability = 0.050), which was not possible with the low-displacement test configuration. Further, the high-displacement testing showed that the more soluble the test chemical was in polyethylene, the lower the resulting delta modulus.