Research in physics education indicates that the use of multiple representations in teaching and learning helps students become better problem-solvers. We report on a study to investigate students' difficulties in solving mechanics problems presented in multiple representations. We conducted teaching/learning interviews with 20 students in a first-semester calculus-based physics course which covered introductory classical mechanics. Each student was interviewed four times during the semester, each time after they had completed an exam in the course. During these interviews, students were asked to solve a problem they had seen in the exam, followed by problems that differed in the type of representation from the exam problems. Students were provided verbal hints to solve the new problems. We discuss the common difficulties students encountered when attempting to solve problems in different representations and some common themes in students' performance. An appendix to this document presents the topics and problems in each interview. (Contains 3 tables and 1 figure.)