OBJECTIVE To present the process evaluation of a curricular Cross-curricular Unit on Portion Size (CUPS) program that integrates nutrition and mathematics, describing teacher and student perspectives on the intervention. METHODS Semi-structured interviews and focus groups were conducted following the implementation of the CUPS program during a pilot randomised controlled trial designed to evaluate efficacy for improved portion size estimation. Lessons involved experiential learning using food models and mathematics cubes and focussed on portion size, food groups, volume and capacity. Data were collected immediately post-intervention and analysed using an inductive thematic approach. METHODS Primary schools in Newcastle, Australia. METHODS Year 3 and/or 4 teachers (n 3) and their students (n 15). RESULTS Teachers believed the programme supported the learning of nutrition concepts, with the majority of students enjoying the lessons, cubes and food models. Teachers indicated most students were engaged and became more aware of healthy eating and serve size recommendation. Although teachers enjoyed and valued the lessons, they suggested that the integration of volume and capacity should be further improved in order to address the time barrier for teaching nutrition. CONCLUSIONS The process evaluation reports on challenges and successes of implementing an integrative nutrition programme. This teaching approach could be useful and successful when aligned with teacher' and student' needs. Based on participant feedback, lessons could be refined to enhance integration of mathematics content and to support student learning.