Peer tutoring in music education is an under-researched but burgeoning area of study. This study aims to assess the effects of the same-age peer tutoring project Ritmos en dos on rhythm reading fluency and comprehension. For this purpose, 24 students from the third grade of secondary education participated in the project. This research utilises a mixed-methods sequential explanatory design, combining (a) a pretest-posttest control group design, with (b) a qualitative study to understand the possible quantitative changes using the interactivity analysis model. Results confirm our hypotheses that students’ participation in the project improved their proficiency across rhythm reading fluency and comprehension. Regarding fluency, quantitative results show a significant improvement for the experimental group, but not for the control group; in comprehension, both groups show gains, but statistically significant differences favour the experimental group in the final comprehension level. Findings reveal different actions that might explain these improvements identified in the interaction between students. Regarding fluency, these actions were mainly related to pulse constancy and reading accuracy while in reading comprehension, students were found to carry out syntactic analysis and metacognitive reflection. Implications suggested from this study point to initial training and cooperative interaction as key elements to be tackled.