Cumulatively, participation in optional science learning experiences in school, after school, at home, and in the community may have a large impact on student interest in and knowledge of science. Therefore, interventions can have large long-term effects if they change student choice preferences for such optional science learning experiences. To be able to track K-12 students' intentions to participate in optional science learning experiences, we developed a new measure of science choice preferences in early adolescence. The present study with 284 5th and 894 6th graders from diverse school contexts (i.e., from the Bay Area and the Pittsburgh area) illustrates the value of applying Item Response Theory analyses to develop a measurement instrument. These analyses established the overall reliability of the instrument and each item in the scale, as well as the generalizability of the scale and individual items across subgroups by gender, by ethnicity, and by achievement levels in science. Further, preferences to participate in science were shown to be separate from preferences to participate in mathematics, engineering, or medicine. Finally, the science choice preferences measure is validated through replicated positive correlations with levels of science interest, self-efficacy, and learning achievement.