This study tested a model of teacher interventions (TIs) conducted during cooperative learning to examine how they affected students' subsequent time on-task (TOT) and problem solving. TIs involved groups of ninth-grade students working on an algebra problem; videotaped lessons were transcribed and analyzed. Results showed that teachers initiated most TIs and typically did so when students were off-task or showed little progress. After TIs, students' TOT and problem solving often improved. Teacher evaluations of student actions had the largest positive effects, sewing as gatekeepers for other teacher actions. Higher levels of teacher help content tended to reduce post-TI TOT, while teacher commands reduced post-TI TOT only when a group grasped the problem situation. In summary, TIs can increase TOT and problem solving, especially if teachers evaluate students' work. (Contains 2 footnotes, 9 tables, and 1 figure.)