Researchers working on activity-based mathematics teaching suggest that the materials deeply shape the design and implementations. Despite this effect, empirical knowledge about how teachers approach the materials in their activity selection, and which features of the materials are decisive in their preferences is quite limited. Hence this study aims to designate material features determinant in teachers’ mathematical activity preferences. This research was designed as a multiple case study and was conducted with three secondary school mathematics teachers. During the data collection process, six consecutive semi-structured interviews were conducted with the participants. While structuring the interview process, multiple activities were prepared with different materials to serve the same gain. The data were analyzed by thematic analysis method. The analyses yielded five distinct features that accounted for teachers’ selection and preferences of mathematical activities with regard to materials: serving to the mathematical gains, functionality, accessibility, being proficient in use and student familiarity. The findings showed that teachers' activity preferences had a complex structure and pointed out that instructional decisions were not only shaped on a pedagogical basis and were not only concerned with students' mathematical development. Based on the evaluations of the teachers, it was concluded that the predictions about the affordances and constraints of the materials were decisive in the activity selection.