This study addresses the measurement of high school students' epistemic knowledge associated with scientific experimentation (EKSE) which concerns "how" scientific experimentation generates knowledge and "why" that knowledge is justified. Based on philosophical, educational standards, and literature analyses, an EKSE construct is characterized as (1) underlying students' decisions and reasoning elicited during experiment design, data collection/measurement, and data analysis/interpretation and (2) progressing in five levels of sophistication: no information, nascent, dogmatic, contextualized, and reflective. This study psychometrically examined the structure of the EKSE construct using the construct modeling approach. An instrument was designed to ask students decisions and reasoning encountered while carrying out the activities embedded in experimental inquiry. High school students took the instrument as a pretest and a posttest to a physics curriculum where they learned to carry out independent scientific experiments. Results indicate that (1) all decisions and reasoning taking place during experiment design, data collection/measurement, and data analysis/interpretation contribute to the EKSE construct, (2) epistemic knowledge develops in the hypothesized order, (3) scientifically aligned decisions are likely to occur when students' epistemic knowledge is at or higher than the dogmatic level, and (4) students' EKSE can improve after engaging in a curriculum that encourages independent scientific experimentation with materials. Implications of these results and further research are discussed.