Contamination of natural habitats including the food chains by chemotoxic and/or radiotoxic f-elements is a substantial threat for human life. Dairy products are an integral part of the daily diet of various human cultures and contain high amounts of proteins with phosphorylated amino acid residues, especially serine. These proteins are responsible for the ability to bind Ca2+ but they can also bind poisonous f-elements. The present paper aims to characterize the metal-binding properties at the molecular level by studying the interactions of O-phospho-L-serine with selected ions in solution as well as in solid precipitates by the combined application of liquid-state and solid-state nuclear magnetic resonance (NMR) spectroscopy, complemented by time-resolved laser-induced fluorescence spectroscopy (TRLFS). In solution, trivalent ions of lanthanum, europium, and lutetium, as well as of the lighter, NMR-active surrogate yttrium, are primarily coordinated upon bidentate phosphate complexation. In the obtained solids the ligand’s carboxylate group is involved in coordination. Depending on solution conditions, three complex species are forming, one of which prevailing as soluble 1:1 complex species and two of which forming solid phases of 3:2 and 1:1 SerP:metal ratio.