Curriculum reform worldwide has often reflected changing perspectives on the teaching and learning of science. Such perspectives underline the notion that not only the teaching of science content knowledge is relevant but also the aims and methods scientists use to further their knowledge and the social context in which science is applied. We call this the 'cognitive-epistemic' and 'social institutional aspects' of science. They form the hard core of the nature of science (NOS) concept for curriculum reform. In South Africa, after years of Apartheid, a key focus of school science curriculum reform is for importance to be given to NOS due to the potential benefits of learning and understanding of NOS for students. This study analyses cognitive-epistemic and social-institutional categories of NOS in the South African Life Sciences and Physical Sciences school curricula. A critical finding for both Life Sciences and Physical Sciences curricula was the imbalance in the NOS categories. In the curricula, there is less presence of the social-institutional categories compared to the cognitive-epistemic categories. In general, there was poor interconnectedness amongst NOS categories, with the strongest interconnectedness revealed for the cognitive-epistemic categories 'scientific practices' and 'methods and methodological rules'. Implications for the design of future curricula and recommendations for future research are discussed.