Assessment determines what students regard as important: if a teacher wants to change students' learning, he/she should change the methods of assessment. This article describes the use of portfolio assessment on five courses dealing with chemical reactor and process design during the years 1999-2001. Although the use of portfolio was a new assessment method for the students, about half of them chose portfolio assessment instead of a traditional examination. The organization of portfolio assessment (the structure of the portfolio, negotiations about the assessment principles between students and teachers, etc.) and students' and teachers' reactions to this new assessment method are described. Portfolio proved to be a good addition to the assessment methods and an excellent way of getting feedback from the students. (Contains 1 table.)