The use of freely-available web-based materials in professional development has rarely been investigated in mathematics education research. In this article, the responses to a survey by 267 preschool teachers about their use of online professional development materials are described. The web materials were based on a design model and the survey results are compared with its principles which focus on content, tasks and relationships. The results show that some of the content in the written materials was considered difficult to understand. Nevertheless, the requirement to work on the material in groups and to carry out tasks in their own preschools supported the teachers to develop a more comprehensive understanding of the content over time. Generally, the teachers found the material valuable for gaining knowledge about mathematics for young children that they could implement as tasks to challenge children in their preschools.