The nature of undergraduates' ideas about thennodynamics and the everyday basis of those ideas was investigated. A sample of fourteen Portuguese undergraduate students in their last year of university studies in physics and chemistry (teacher training) were interviewed individually about five chemical phenomena. The results showed that these students found it very difficult to use thermodynamic concepts to discuss real situations. The physical reality of the situation seemed to dominate thinking. The majority of the students used criteria based mainly on observable features or everyday notions. There was very little spontaneous use of thermodynamic concepts. It was found that non-science conceptions were persistent despite formal instruction in advanced chemistry the students had received. The second experiment investigated the everyday origin of the ideas the students used and how everyday experiences and meanings and formal instruction may have influenced these ideas. It was carried out by interviewing individually ten secondary school pupils (9th and 12th grades) and five adults. The results showed that the intuitive way of thinking about the physical world influences strongly what students learn about 'new', 'abstract' and 'scientific' ways of explaining what happens in nature. The comparison of the results of both experiments showed that there was close similarity between pupils' and university students' ideas. Several reasons were put forward as explanation: (i) students are more confident using criteria based on perceivable features (ii) the context of real phenomena makes the use of theoretical knowledge difficult (iii) students spend more time dealing with 'everyday meanings' than with 'scientific meanings' (iv) the concepts are abstract and (v) science uses words also used in ordinary speech. Implications for teaching are pointed out.