The chemical composition and morphology of alkali-silica reaction (ASR) products is of great importance in studying the reaction mechanism and assessing the effectiveness of mitigation techniques. Epoxy-impregnated, polished thin sections were prepared from an in-service concrete pavement to examine ASR products that originate from chert particles in the fine aggregate. Both hydrous and anhydrous thin-section preparation techniques were used to investigate the influence of sample preparation on the results. Two distinct morphologies of ASR products were observed in the voids adjacent to the chert particles: a bladed crystalline type and a glassy amorphous type. The chemical compositions of the reaction products were determined with a scanning electron microscope via quantitative X-ray energy dispersive spectrometry. Both the chemical composition and morphology were influenced by sample preparation, with hydrous preparation resulting in leaching and degradation. It is recommended that, when conducting studies to assess the ASR mechanism and the effectiveness of mitigation techniques, anhydrous sample preparation be used.