In response to the demand for sound science assessments, this article presents the development of a latent construct called knowledge integration as an effective measure of science inquiry. Knowledge integration assessments ask students to link, distinguish, evaluate, and organize their ideas about complex scientific topics. The article focuses on assessment topics commonly taught in 6th- through 12th-grade classes. Items from both published standardized tests and previous knowledge integration research were examined in 6 subject-area tests. Results from Rasch partial credit analyses revealed that the tests exhibited satisfactory psychometric properties with respect to internal consistency, item fit, weighted likelihood estimates, discrimination, and differential item functioning. Compared with items coded using dichotomous scoring rubrics, those coded with the knowledge integration rubrics yielded significantly higher discrimination indexes. The knowledge integration assessment tasks, analyzed using knowledge integration scoring rubrics, demonstrate strong promise as effective measures of complex science reasoning in varied science domains. (Contains 5 tables, 4 figures, and 1 footnote.)