Adoption of new instructional standards in science demands high-quality information about classroom practice. Teacher portfolios can be used to assess instructional practice and support teacher self-reflection anchored in authentic evidence from classrooms. This study investigated a new type of electronic portfolio tool that allows efficient capture of classroom artifacts in multimedia formats using mobile devices. We assess the psychometric properties of measures of quality instruction in middle school science classrooms derived from the contents of portfolios collected using this novel tool--with instruction operationalized through dimensions aligned to the Next Generation Science Standards. Results reflect low rater error and adequate reliability for several dimensions, a dominant underlying factor, and significant relations to some relevant concurrent indicators. Although no relation was found to student standardized test scores or course grades, portfolio ratings did relate to student self-efficacy perceptions and enjoyment of science. We examine factors influencing measurement error, and consider the broader implications of the results for assessing the validity of portfolio score interpretations, and the feasibility and potential value of this type of tool for summative and formative uses, in the context of large-scale instructional improvement efforts.