In response to the need for studies that focus on learning and writing in mathematics, this study examined changes in students' probabilistic thinking and writing during an instructional program that emphasized transactional writing in a problem-solving context. Although correlations between probabilistic thinking and writing levels at the end of the study were not significant, students did make significant gains in both probability reasoning and writing. Analysis of target students' journals revealed that their writing incorporated both writing symbols and mathematical symbols. These symbols were more complementary for those students whose writing increased to the higher levels during instruction. Moreover, this growth appeared to be promoted by the teacher herself, who systematically sought verbal explanations of solutions and written interpretations of diagrams and numerical patterns.