The aim of the study was to describe, categorise and analyse students' (aged 14-16) processes of scientific inquiry in biology and chemistry education. Therefore, a theoretical structure for scientific inquiry for both biology and chemistry, the VerE model, was developed. This model consists of nine epistemological acts, which combine processes of scientific thinking and inquiry methods. Based on the theoretical structure, a paper-and-pencil test was developed to investigate the students' abilities in the acts of scientific inquiry. Each of the nine acts was operationalised to generate multiple-choice items. For each act, ten items were constructed. In total, ninety items per subject were tested in a field study to evaluate their psychometric quality. The article focuses on the outcomes for testing in biology. In biology, 537 students were tested with a paper-and-pencil test, following a multi-matrix design in which each student solved twenty-seven items. Data from 260 students have been analysed so far. Seventy-five items showed satisfactory item characteristics. The distribution of the items' difficulties fits the students' abilities appropriately. We conclude that theory-driven epistemological acts can be operationalised in tasks that assess students' abilities in scientific inquiry.