Here, we introduce the principle of the novel in situ luminescence analysis of coordination sensor (ILACS) approach for monitoring the formation of solid materials, recording information from the formed solid compounds as well as from the surrounding solutions. This technique utilizes as a main tool the sensitivity of luminescence properties of lanthanide (Ln) ions on the coordination environment, being incorporated as local sensors by the investigated material during synthesis. The luminescence spectra and their environment-dependent developments are monitored in situ from the early stages of the reaction until the final product formation under real conditions with a high time resolution. The ILACS principle is demonstrated here for monitoring the formation of [Eu(phen)2(NO3)3] (phen = 1,10-phenanthroline) and further metal-ligand exchange processes during its conversion to [Sn(phen)Cl4]. These reactions were followed, for instance, analyzing the antenna effect, shift of the (5)D0→(7)F4 Eu(3+) transition and quenching effects. In addition, these results have been validated by comparison with other in situ techniques. The results demonstrate that ILACS is a new powerful, fast, broadly available in situ characterization method, which is applicable for liquids, amorphous samples, and very small crystallites besides for large crystals.