Among the main changes in daily life that are due to the use of online tools, the search for information to help the purchasing decision process was one of the most impacted. Before the popularization of Internet use, positive and negative information about products and services was obtained in small groups of individuals. After popularization, the interactivity between users grew and extended to consumer practices, generating a word-of-mouth communication in electronic media. In this way, this article aims to assess comparatively the influences of the positive and negative electronic mouth-to-mouth communication in the online purchasing decision process. For this, 248 questionnaires were applied and ten hypotheses were verified. As analytical methods were used the descriptive statistic and the confirmatory factorial analysis by means of structural equations. Only the evaluation of alternatives, purchase and post-purchase behavior are influenced.