Detailed analysis of reaction mechanisms by in situ techniques are important for detecting metastable intermediates, analysing polymorphic transitions and thereby for the discovery of new compounds. This article presents the first combination of serial crystallography with in situ luminescence and X-ray diffraction (XRD) measurements to monitor the synthesis of [Eu(phen)2(NO3)3] (phen = 1,10-phenanthroline). In a batch reaction, it is found that this complex is polymorphic, crystallizing into two distinct monoclinic structures. We track the evolution of the synthesis conditions for these phases using in situ XRD combined with real time measurements of pH value, ion conductivity, infrared (IR) spectroscopy and in situ luminescence analysis of coordination sensors (ILACS). However, in a flow reactor a different combination of phases is produced. A serial crystallography experiment utilizing a nanofocused synchrotron X-ray beam to identify individual crystallites reveals the simultaneous formation of the two phases, as well as, a third unknown phase. This showcases the feasibility of phase detection on an individual crystallite level to track the synthesis of new materials.