The aim of this paper is to illustrate, by way of a few case studies, the value of model-based task analysis in interaction and design specification of Web mashups. Task analysis is recognized as one fundamental way to focus on the specific user needs and to improve the general understanding of how users may interact with a user interface to accomplish a given goal when using an interactive system. It is argued that some of current challenges to the design and development of Web mashups are commonplace when building task models. So that, in some extension, model-based task analysis can help designers of Web mashups to better understand and describe users’ tasks for combining data sources. The cases studies presented hereafter focused on tasks carried out by both developers and end-users of Web mahsups. More than simple illustrations, all cases studies are issued from real life applications.