This case study examines the performances of 18 10th-grade students (age 15-16 years) in the process of performing problem-solving tasks in the physics and chemistry laboratory. The study focuses on different types of problems arising in the process of transferring responsibility to students in a context of teacher assistance to autonomous problem-solving. The students' conversations were audiotaped and videotaped, and their productions collected. Problems were found in relation to excessive task difficulty, to stereotyped school culture reflecting procedural display rather than genuine problem-solving, and to problems related to within-group interactions and roles. The findings are discussed in reference to the educational goal of students progressively assuming responsibility in their own learning. (Contains 4 tables and 1 figure.)