The development of deeper understanding and transferable skills in science requires continuous interactive discussion and feedback and extended practice in various contexts for generalisation. In primary schools, these desiderata are difficult to supply through direct teacher instruction, but might be feasible through interactive peer tutoring. This study aimed to evaluate cognitive and affective gains from cross-age peer tutoring for both tutees and tutors in science, using the "paired science" programme. For the first time this programme was used to focus on peer rather than parent tutoring and on junior school rather than early years pupils, and for the first time an objective measure of cognitive gain in science was used. Experimental peer tutees were a whole class (n32) of seven- to eight-year-olds; tutors a whole class (n33) of eight- to nine-year-olds in the same school. A parallel composite class of seven- to nine-year-olds (n24) served as controls for tutees and tutors. A peer tutoring paired science intervention was implemented for two 30-minute sessions per week for eight weeks. Video and observational data indicated implementation integrity was satisfactory. Pre- and post-project assessments of understanding of scientific concepts and keywords of a random sample of tutees, tutors and their respective controls (4n10) were conducted. Additionally, the attitudes of all the tutees, tutors and their teachers towards their experiences were explored by post hoc questionnaire. On pre-post assessments of understanding of scientific concepts and keywords, the experimental group made significant gains while the control group made no gains, yielding effect sizes greater than one, Tutees made greater gains than tutors. The attitudes of the paired scientists and their teachers towards their experiences were generally very positive. It was concluded that cross-age peer tutoring of science using the paired science programme offers an effective pedagogical strategy, with both cognitive and affective benefits for both tutors and tutees. Recommendations for future research are made.