Scientific reasoning is a crucial ability in modern knowledge societies. An increasing body of developmental and educational research shows that already primary-school children possess emerging scientific-reasoning skills. The Science-P Reasoning Inventory (SPR-I) is a novel classroom-based paper-and-pencil instrument that was developed to measure children's conceptual advances across diverse components of scientific reasoning (experimentation, data interpretation, understanding the nature of science). The SPR-I comprises 19 items, and a reduced, 7-item version is available: the SPR-I(7). Both versions measure broad scientific-reasoning skills and they can be used for whole-class testing. The administration of the SPR-I(7) requires only 30 min, allowing for its use across diverse research settings and the economic assessment of large samples of primary-school students. This article reports on the first empirical test (IRT analysis) of the theoretical, conceptual-development model that guided the construction of the SPR-I. Both the SPR-I and the SPR-I(7) are available from the Supplementary Materials.