The nature of external surface barriers in the zeolite ZSM-5 was explored by comparing mass transfer of n-heptane in samples with different crystal sizes, surface structure, and composition. These different surface properties were achieved by HF acid etching and SiO2 deposition on an as-synthesized ZSM-5 crystal sample. HF etching does not reduce surface barriers. SiO2 deposition on the same sample reduces surface barriers significantly. These insights into the nature of surface barriers could be used to guide the rational design of zeolitic materials with better mass transport properties.