Summary.This investigation showed that a concept of volume conservation can be taught to educationally subnormal children and that this understanding of conservation generalises to weight and substance situations. 104 ESN school children were pre‐tested for conservation of substance, weight and volume. Then, using a varied teaching method which included three conservation of volume experiences, reasons, and situations, 30 of these children (non‐conservers aged 9 to 15 with IQs between 46 and 75) were taught to recognise, generalise and give reasons for conservation. Post‐testing after one and two weeks and again after five months revealed that their concept of conservation was durable as well as generalised. The teaching method used was found to be an effective way, though not necessarily the only or the best way, of developing conservation in children regardless of their prior understanding of conserv