Background/Question/Methods Processes that transform carbon (photosynthesis, oxidation, biosynthesis) play a prominent role in college biology and ecology classes. Furthermore, an understanding of the carbon cycle is a critical component of environmental literacy. Our goals were to learn about student reasoning related to carbon-transforming processes and to provide faculty with tools to assist with instruction and assessment. We created a conceptual framework to illustrate how carbon-transforming processes could be related to one another during instruction by explicitly teaching students to use principled reasoning: to use the Laws of Conservation of Energy and Matter and to identify the appropriate scale at which to reason. Frameworks such as ours may improve biology instruction more effectively than simply cataloging alternate conceptions and addressing them individually. Using an iterative process of pilot testing, student interviews, and revision, we created four sets of diagnostic question clusters (DQCs) designed to help college faculty assess their students' understanding of carbon-transforming processes from atomic-molecular through ecosystem scales. Each cluster included multiple choice, open response, and true/false questions and could be completed in 15-20 minutes. Data presented here are compiled from seven U.S. universities, at which faculty administered pre and post-tests to their students. Results/Conclusions 1. Problems with student understanding were prevalent in all grade levels at all types of institutions. Even students taking advanced courses relied on everyday cultural models rather than principled scientific reasoning to answer most questions. 2. Faculty participants' teaching, including the active learning strategies they employed (see other posters in this session for details), made a difference. The percentage of students with responses indicating principled reasoning more than doubled on post-tests compared to pre-tests. However, more than 70% of students answered posttest questions at levels below the highest standard set in our response coding strategy. 3. Our results both corroborate and extend previous research about student misconceptions. 4. Multiple misconceptions seemingly have the same root cause, such as the failure to conserve matter. For example, in a question about photosynthesis, 57% (n=249) of students thought atoms in chlorophyll could come from sunlight. In a question about respiration, 67% (n=176) of students thought amylose (CHO) could be broken down to N and P during decomposition of a potato. Although the questions are about different processes (growth and decomposition) results reveal common difficulties students have integrating their knowledge at different scales, which is further hindered by basic misconceptions about matter and energy.