Teachers' involvement in curriculum design is essential for sustaining the relevance of technology-enhanced learning materials. Customizing--making small adjustments to tailor given materials to particular situations and settings--is one design activity in which busy teachers can feasibly engage. Research indicates that customizations based in evidence from student work lead to improved learning outcomes. In this paper, we examine the customizations of four middle and high school teachers during their enactments of web-based inquiry science units. We examine how specific technology features afforded their customizations by providing tools for making adaptations and by making student work available as evidence for those adaptations. Cases built from classroom video and field note observations, interviews, and teachers' curriculum artifacts, revealed three kinds of customizations: (a) devising timely instructional interventions to provide individualized guidance; (b) planning activities and adjusting milestones to align with students' progress; (c) modifying existing materials to better integrate content into overall curriculum plans; and (d) incorporating scaffolds to better address students' needs. We also identified three technology features that supported teachers' customizations: (1) a system that logs student work for teachers' inspection; (2) tools for conducting dynamic, formative assessment; and (3) an authoring environment that supports the re-design of units at multiple levels of granularity. We end by suggesting design principles for curriculum materials that support teachers' customizations, as well as future directions for technology that would enhance teachers' participation as designers.