This paper aims to study children's thermal comfort and related Adaptive Behaviours in UK primary schools. The study was carried out in 32 naturally-ventilated classrooms during Non-Heating (NH) and Heating (H) seasons. Alongside collecting environmental data, a self-reported questionnaire and an observation form were employed to record children's thermal comfort and adaptive behaviours. From eight primary schools, 805 children aged 9-11 were surveyed and 1390 questionnaires were collected. Children's Thermal Sensation Votes (TSVs), Thermal Preference Votes (TPVs) and adaptive behaviours were compared against temperature offset from comfort temperature by EN 15251 (T-diff=T-op-T-C(CEN)). Results suggest that children's thermal comfort (T-C(children)) is 1.9 K and 2.8 K lower than that for adults (T-C(CEN)) during non-heating and heating seasons, respectively. Children have lower comfort temperature and higher sensitivity to temperature changes during heating seasons. This can be attributed to children's lower practice of personal behaviours and more consistent indoor conditions during heating seasons. The proportion of children engaged with personal behaviours is one-third lower during heating seasons. As indoor temperature goes above children's thermal comfort band, the proportion of children practising personal behaviours increases during non-heating seasons. Around 80% of window operation is carried out by teachers who have a higher comfort temperature than children. (C) 2020 Elsevier B.V. All rights reserved.