This paper develops and validates a theoretical extension of the Technology Acceptance Model (TAM). The extended model aims to predict and explain consumers’ intentions to transact with an Internet-based business-to-consumer electronic commerce (B2C EC) system by integrating trust and risk perceptions with TAM. Trust is the foundation of commerce. Its influence on business relationships is even greater in the online environment, where there are no face-to-face interactions between the transacting parties. Under these conditions, consumers' uncertainty on the transaction outcome would increase, and trust plays an important role in their decision to transact online. In this study, perceived risk is proposed to be a direct antecedent of intention to transact, and the various dimensions of trust are proposed to have a positive influence on perceived risk. Besides testing the model, the relative importance of the trust dimensions is also examined. The model was validated using data collected from 133 subjects. The results provided substantial support for most of the proposed hypotheses and showed the significance of the extended constructs. Several new insights on trust in B2C EC were found and their theoretical implications are discussed.