A mobile eye tracker was used to collect viewing behavior in a mixed reality immersive Cave Automatic Virtual Environment (CAVE) environment to evaluate a design concept of a tourist information office. The synthetic office consists of physical artifacts and virtual contents projected onto three walls of a room-sized cube. A Think Aloud study was conducted with both a goal-oriented condition and a free-browsing condition while subjects wearing the eye-tracker. Multiple Augmented Reality markers were used to reconstruct gaze positions in the coordinate system of the real environment. Gaze points were later aggregated to create heat maps, which were used as textures for a computer 3D model replication of the synthetic tourist office. The interactive visualization of the 3D heat map showcases different viewing patterns for different conditions. The insights suggest the combination of eye-tracking and mixed reality environment to be a valuable tool for prototyping service design of similar kinds.