*The literature on the willingness to bid and the actual bidding behaviour of consumers in online auctions is currently dominated by approaches based on the economic decisionmaking and information processing paradigm and are primarily focused on what influences auction outcomes. To the best of our knowledge, no serious attempts have been undertaken to stringently test and compare existing models derived from an actiontheoretical perspective to predict and explain consumers´propensity to use online auctions as well as their actual usage. Two theoretical models seem most promising in this context: The Theory of Planned Behaviour (Ajzen I, 1985(Ajzen I, , 1991 and a derivative of the Theory of Reasoned Action (Fishbein M and Ajzen I, 1975) tailored towards using computer technologies, the Technology Acceptance Model (Davis FD, 1989). In both theories, intentions play a central role in predicting behaviour. The models differ in their descriptions of the factors that determine behavioural intentions. In the Theory of Planned Behaviour, attitudes towards the behaviour, perceived behavioural control and subjective norms are assumed to influence intentions. In contrast, the Technology Acceptance Model suggests that intentions and attitudes are influenced by the perceived usefulness of a certain technological tool to improve shopping productivity (e.g. by enabling the consumer to obtain a better price or save time) and the tools´perceived ease of use. In principle, both theories can be used to predict and explain technology-dependent consumer behaviour, but which one is more suited to online auctions? We compared both theories in terms of their predictive power and their practical utility. Although both models explain the propensity to bid in online auctions very well, the Technology Acceptance Model provides more specific recommendations for facilitating the use of online auctions.