Space-filling techniques have been used in the information visualization field as an alternative to the conventional node-link layouts for intuitively showing large hierarchies in less space. Different space filling layouts have been designed, developed and evaluated; however, much less effort have been made to look into how layout can impact user task performance on hierarchical data structures . In this paper, we focus on the impact of layout on user task performance by conducting eval uation studies for two common space-filling layout structures, the Sunburst (radial) layout and the Icicle (rectangular) layout. In our studies, users performed eight search -based tasks on files and directories in the resulting visualizations, first in a controlled environment and subsequently in an online environment. We focused on deriving user performance metrics with regard to effectiveness, efficiency, and user acceptance. Results demonstrate a mixed view of task performance and preference with both la youts, e.g., users performed better with the Icicle layout while they preferred the Sunburst layout for visual aesthetics. We further analyzed the impact of layout on the performance dynamics in terms of response times and accuracy using event history analysis (EHA) in the control study setting. The EHA results revealed clear differences in response tendencies even though no differences existed in mean response times for most of the tasks. It also clearly showed that participants performed more efficiently with the directory comparison tasks than the file comparison tasks. Overall, through these studies we were able to derive causal relationships between the layout and the user’s task performance while interacting with hierarchical data structures.