This paper describes a series of user studies on how people use the Web via mobile devices. The data primarily comes from contextual inquiries with 47 participants between 2004 and 2007, and is complemented with a phone log analysis of 577 panelists in 2007. We report four key contextual factors in using the Web on mobile devices and propose mobile Web activity taxonomy. The framework contains three user activity categories identical to previous stationary Web studies: information seeking, communication, and transaction, and a new category: personal space extension. The new category refers to the practice that people put their content on the Web for personal access, therefore extending their personal information space. Categories and Subject Descriptors H.5.2 [User Interfaces]: User-Centered Design General Terms Design, Measurement, Performance, Human Factors, Theory. Keywords Mobile Web, Activity Taxonomy, Information Seeking, Content Object Handling, Personal Space Extension.