Online consumer reviews and expert reviews play a crucial role in customers’ purchase decisions. Existing studies explored the influence of consumer reviews and expert reviews on purchasing decisions separately. No research has examined how expert reviews and consumer reviews affect purchasing behaviors simultaneously. This study adopted the event-related potentials (ERP) technique to investigate the underlying cognitive process and purchasing behaviors when confront with different portfolios of consumer reviews and expert reviews. Participants were required to make a purchase decision according to the presented stimuli in each trial. Behavioral results indicated that a higher purchase rate was observed when expert reviews and consumer reviews are positive, followed by the portfolio of positive expert reviews and negative consumer reviews. The results suggested that expert reviews outweigh consumer reviews when making purchase decisions. At the neural level, when expert reviews and consumer reviews are consistent (both positive or negative), positive reviews could evoke smaller P2, larger N2, and larger later positive potential (LPP). When expert reviews and consumer reviews are conflictive, the portfolio of positive expert reviews and negative consumer reviews could elicit smaller P2, N2, and larger LPP. The physiological results revealed a three-stage cognitive pattern before making purchase decisions, including early attentional resources allocation with P2, conflicts perception with N2, and the later evaluative categorization with LPP. Briefly, these findings provided a preliminary exploration of consumers’ underlying cognitive processes and neural responses when making purchase decisions. Our findings will help marketers better understand consumers’ cognitive processes and contribute to managing online reviews.