Test items used to survey the attainment of science process skills in a representative sample of 11 year olds in Britain were analyzed to determine the level of demand, in Piagetian terms, made by each item. It was found that: the level of cognitive demand was a reliable predictor of the limiting difficulty of an item; that the difference between potential difficulty as measured by supposed demand level and the actual difficulty encountered by subjects was revealing of areas in the curriculum where further effort in teacher education and curriculum development would be profitable; and that there was some evidence for an inherent hierarchy of process skills.