We examined the structure of domain-specific academic science vocabulary, general academic vocabulary, and conceptual science knowledge and their impact on each other across two measurement points in a sample of 388 German third graders. Results show that the three constructs can be empirically differentiated. The high latent correlation between science vocabulary and general academic vocabulary suggests that science vocabulary is a language-related rather than a knowledge-related construct. Furthermore, science knowledge and science vocabulary predicted each other. By contrast, general academic vocabulary was only associated with science vocabulary but not with science knowledge acquisition. Neither science knowledge nor science vocabulary predicted general academic vocabulary at the end of the school year, suggesting it might be less sensitive to regular instruction.