We investigated a theoretical model including an instructional intervention and systematic processing to account for change in preservice teachers' epistemological beliefs about teaching and learning in mathematics. General and subject-specific epistemological beliefs and systematic processing were assessed in 161 preservice teachers, randomly assigned to an experimental group whose epistemological beliefs about mathematics were activated and challenged through augmented activation and refutational text or to a control group who read a traditional expository text. The model was partially supported. The treatment group receiving the instructional intervention demonstrated greater change in implicit epistemological beliefs than the control group, and partial support for systematic processing as a mediator of the relationship between general epistemological beliefs and change in specific epistemological beliefs was obtained.