An attempt to characterize the interaction of rare earth elements with P507 in a microfluidic extraction system containing lactic acid as complexing agent is made using spectroscopic (FT-IR, UV-Vis, NMR and MS) analysis. The role of lactic acid in the extraction process is analyzed. The extraction mechanism confirmed that lactic acid does not involve in the extraction reaction. A comparative analysis of the composition of the extracts using FTIR, UV-Vis, NMR and MS methods, indicates that the extracted complexes of rare earth ions located in the center is formed by the cation exchange process between P-O-H and RE3+, and coordination process between P=O and RE3+.