Given the central importance of the Nature of Science (NOS) and Scientific Inquiry (SI) in national and international science standards and science learning, empirical support for the theoretical delineation of these constructs is of considerable significance. Furthermore, tests of the effects of varying magnitudes of NOS knowledge on domain-specific science understanding and belief require the application of instruments validated in accordance with AERA, APA, and NCME assessment standards. Our study explores three interrelated aspects of a recently developed NOS instrument: (1) validity and reliability; (2) instrument dimensionality; and (3) item scales, properties, and qualities within the context of Classical Test Theory and Item Response Theory (Rasch modeling). A construct analysis revealed that the instrument did not match published operationalizations of NOS concepts. Rasch analysis of the original instrument--as well as a reduced item set--indicated that a two-dimensional Rasch model fit significantly better than a one-dimensional model in both cases. Thus, our study revealed that NOS and SI are supported as two separate dimensions, corroborating theoretical distinctions in the literature. To identify items with unacceptable fit values, item quality analyses were used. A Wright Map revealed that few items sufficiently distinguished high performers in the sample and excessive numbers of items were present at the low end of the performance scale. Overall, our study outlines an approach for how Rasch modeling may be used to evaluate and improve Likert-type instruments in science education. (Contains 1 note, 4 tables, and 3 figures.)