This study focuses on learning about the effect of a peer tutoring program on the development of reading comprehension. The study comprised 127 students in the intervention group, along with 75 students in the comparison group, all of them enrolled in 4 primary schools. The study also involved 8 teachers, who were responsible for modulating the sessions. A quasi-experimental study was carried out, which included a comparison group, combined with analysis of the working sessions of a sub-sample of 12 pairs, to learn about the usual actions of students during the activity. The obtained results show statistically significant progress by the intervention group, but not by the comparison group. Improvement in reading comprehension affected all students, with higher effects on tutors or participants who carried out both roles in reciprocal tutorials. Analysis of the interaction shows that peer tutoring and organization of peer interaction has allowed students to learn due to the structuring of the help received (prompting and linking of ideas), in the case of the tutorials, or due to the learning by teaching strategy in the tutors. (C) 2015, Konrad Lorenz University Foundation. Published by Elsevier Espana, S.L.U. This is an open-access article distributed under the terms of the Creative Commons CC BY-NC ND Licence (http://creativecommons.org/licenses/by-nc-nd/4.0/).