Although Pedagogical Content Knowledge (PCK) is traditionally defined as a static quality that teachers possess and apply in practice, we conceive of PCK as constituted by a dynamic, mutually influencing process between teacher and students. This co-constructing process is expressed in real-time interaction and defined by us as Expressed Pedagogical Content Knowledge (EPCK). The aim of this study is to develop a practically usable instrument that can track the microgenetic moment-to-moment interaction that embodies EPCK, and that enables us to observe different features and levels of EPCK in the form of changes in this interaction dynamics. We were interested to know how EPCK emerges and develops on the short-term time scale of classroom interaction. After presenting a general account of complex dynamic systems based measurement of psychological constructs (e.g., PCK), we describe a coding scheme for teacher-student interactions, based on theoretical EPCK components. The instrument was applied in an empirical observation study of a visit to a mobile planetarium by a grade 3 primary school class. A principal factor analysis was used to find latent EPCK components. Results show, firstly, that the instrument was reliable. Secondly, the variables in the coding scheme were relevant in view of the underlying theory. Thirdly, over the time course of the teaching session, latent components displayed various levels of EPCK – high, low or no ECPK. Instead of being an enduring or stable property of teaching-learning interactions, EPCK is a dynamic property occurring in the form of sequences of high and low levels, and corresponding peaks in the latent factors. Notably, EPCK did not appear in the form of a continuous steady state level but occurred in the form of bursts of high-level EPCK. We conclude that our coding scheme provides an adequate method for studying pedagogical content knowledge as it self-organizes in the form of real-time activity in the classroom.