Nielsen’s usability heuristics are the most commonly used assessment tool to perform a heuristic evaluation. However, when they are employed to evaluate the usability of transactionalWeb sites, they fail to cover all aspects of usability that are currently present in this kind of software. For this reason, we have developed a new proposal that is capable of providing more accurate results in this context. Our approach includes fifteen usability heuristics for the design of usable graphical user interfaces. This paper presents a comparative study between the classical Nielsen’s proposal and the new set of heuristics for transactional Web sites. The experimental case study was conducted following the Method Adoption Model which establishes the analysis of three dimensions: perceived ease of use, perceived usefulness and intention to use. For this purpose, forty-six undergraduate students were asked to perform a heuristic evaluation, in which both proposals were employed. The results showed that the new heuristics are easier to use than the traditional approach. This study provides the validation of an effective tool that can be used easily to perform heuristic evaluations in the context of transactional Web applications.