Purpose - To understand the effectiveness of electronic word of mouth, the purpose of this paper is to examine how high- vs low-knowledge consumers judge and attribute the credibility of positive and negative online reviews by drawing upon accessibility-diagnosticity theory and attribution theory. Design/methodology/approach - This study conducts an observation-based study in an online forum and a 2 (review valence) x 2 (consumer knowledge) between-participants factorial experiment to examine the proposed hypotheses. Findings - High-knowledge consumers elicit less perceived credibility and make more non-product-relevant attribution than low-knowledge consumers in negative online reviews. Consumer attribution is also found to mediate the effects of the review valence by consumer knowledge interaction on review credibility. Originality/value - This study adds to extant research by examining how consumer knowledge plays a key role in determining consumer perception of online review credibility. This study also advances the understanding of different casual inferences about online reviews between high- and low-knowledge consumers.