: Digital services can be thought as internet based applications that fulfil users' needs and their quality represents a basic element during the delivery process. In such a context, usability is one the main quality parameters and it refers to the ease of benefiting from the functionalities and the information the service provides. We propose a list of quality parameters and a formal model suitable for the quality assessment of digital services. In order to examine the effectiveness of our model, we report the results of an experimentation that we carried out to study the quality of a digital e-government service.