The purpose of this study was to validate a new inventory designed to measure epistemic beliefs concerning the justification of knowledge claims encountered on the Internet. Confirmatory factor analysis of the scores of 394 preservice teachers showed that a three-factor model including justification by authority, justification by multiple sources, and personal justification fit the data well. Moreover, latent variable structural equation modeling showed that these three types of justification beliefs differentially predicted aspects of Internet-specific reading motivation and engagement. Taken together, these results provide evidence for the construct validity of the inventory and lay the foundation for further validation work. The theoretical and educational significance of the study is discussed, and directions for future research are suggested.