This six-year study involved an evaluation of whether student-learning gains for an online homework system that emphasized areas in which students are known to have difficulties--molecular-level conceptualization and visualization, and being able to recognize and relate a concept when it is represented symbolically, numerically, or visually--could be sustained when scaled up to multiple sections and multiple instructors over several years. Thirteen sections of the same course were examined with 3,806 students taught by five different instructors. Results showed that the online homework led to higher achievement scores on final exams. Even when adjusted for students' level of preparation coming into the class, the online homework substantially influenced exam performance. Several mechanisms for the positive use of online homework by faculty and students, and issues of scaling up to the departmental level, are discussed. (Contains 4 tables and 2 figures.)