Many websites of online retail stores display two forms of electronic word of mouth on a single product site, namely single customer reviews and aggregate review scores. This study compares the effects of simultaneously presented single customer reviews and aggregate review scores on product evaluations of visitors of online retail stores. We hypothesize that well-argued single customer reviews should be more influential than aggregate review scores that display the mean rating of all published reviews of a product. Drawing on a 2 × 2 factorial online experiment, we show that a single customer review is effective, even when potential buyers do not perceive it as representing overall customer satisfaction. Effects of the aggregate review score are weaker and completely mediated by a potential buyer's estimate of overall customer satisfaction. Trust in product reviews as a credible source of product-related communication moderates some of these effects. Copyright © 2014 John Wiley & Sons, Ltd.