In a longitudinal study conducted in Reykjavik, Iceland, 60 male and 61 female subjects were tested at 7, 8, and 9 years of age in order to investigate their comprehension of three concrete-operational concepts (conservation, class inclusion, and logical multiplication). This report focuses on a logical reconstruction of the developmental sequence of the three concrete-operational concepts. Based on a structural task analysis, a developmental model was postulated that defines the emergence and consolidation of concrete operations as a successive (cumulative) process. Empirically, all children in the longitudinal study followed an invariable developmental sequence corresponding to a unidirectional model. Methodological implications of the longitudinal design and the logical formulation of multilateral developmental models are discussed and an empirical evaluation and statistical test of the developmental model is provided. It is concluded that the findings may be viewed as additional theoretical and statistical evidence for the decalage hypothesis of the development of concrete operations. (Author/RH)