BACKGROUND: According to the language expectancy theory and the communication accommodation theory, health information seekers’ trust evaluations of Web-based videos are determined by interplays between content and seekers’ expectations on vloggers’ appropriate language use in specific contexts of Web-based communication. OBJECTIVES: Two investigations focused on differences both between vloggers’ language styles and between users’ general trust in specific Web-based platforms to investigate how the context of Web-based communication can be characterized (research question, RQ1). Thereafter, we investigated whether information uncertainty, vloggers’ language style, and context of Web-based communication affect seekers’ trust evaluations of videos (RQ2). METHODS: With a content analysis of 36 health videos from YouTube and Vimeo, we examined the extent of trust-related linguistic characteristics (ie, first-person and second-person pronouns). Additionally, we surveyed participants (n=151) on their trust in YouTube and Moodle (academic Web-based platform; RQ1). In an experiment, further participants (n=124) watched a video about nutrition myths and were asked to evaluate the information credibility, vloggers’ trustworthiness, and accommodation of language by vloggers (RQ2). Following a 3 × 2 × 2 mixed design, vloggers’ explanations contained unambiguous (confirming or disconfirming) or ambiguous (neither confirming nor disconfirming) evidence on the myths (within factor). Furthermore, vloggers used YouTube-typical language (many first-person pronouns) or formal language (no first-person pronouns), and videos were presented on YouTube or Moodle (between factors). RESULTS: The content analysis revealed that videos on YouTube contained more first-person pronouns than on Vimeo (F1,35=4.64; P=.04; ηp2=0.12), but no more second-person pronouns (F1,35=1.23; P=.23). Furthermore, when asked about their trust in YouTube or Moodle, participants trusted YouTube more than Moodle (t150≤−9.63; all P ≤.001). In the experiment, participants evaluated information to be more credible when information contained unambiguous rather than ambiguous evidence (F2,116=9.109; P<.001; ηp2=0.14). Unexpectedly, information credibility did not depend on vloggers’ language style or the video platform (F1,117≤2.40; P ≥.06). Likewise, video’s platform did not affect participants’ evaluations of vloggers’trustworthiness (F1,117<0.18; P>.34). However, participants judged vloggers who used a YouTube-typical language as being more benevolent, and their language use as being more appropriate in both video platforms (F1,117≥3.41; P ≤.03; ηp2≥0.028). Moreover, participants rated the YouTube-typical (vs formal) language as more appropriate for Moodle, but they did not rate one or the other language style as more appropriate for YouTube (F1,117=5.40; P=.01; ηp2=0.04). CONCLUSIONS: This study shows that among specific Web-based contexts, users’ typical language use can differ, as can their trust-related evaluations. In addition, health information seekers seem to be affected by providers’ language styles in ways that depend on the Web-based communication context. Accordingly, further investigations that would identify concrete interplays between language style and communication context might help providers to understand whether additional information would help or hurt seekers’ ability to accurately evaluate information.