The complexation reactions between strontium (Sr2+) and octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) were studied at the aqueous|1,2-dichloroethane (w|DCE) and aqueous|room-temperature ionic liquid (w|RTIL) microinterfaces, in order to understand its extraction in reprocessing spent nuclear fuels, remediation of environmental contamination, and potential radiological isotope feed stock for 90Y from its isotope 90Sr in fission byproducts. The stoichiometry (or metal to ligand ratios) and overall complexation constant (β) for these reactions at these two interfaces are described herein. Two stoichiometries at the w|DCE interface were discovered, that is, [Sr(CMPO)2]2+ and [Sr(CMPO)3]2+ with β values of 4.5×1019 and 5.5×1025, respectively. Only one stoichiometry was observed at the w|RTIL interface: [Sr(CMPO)3]2+ with β equal to 1.5×1034. The larger complexation constant for [Sr(CMPO)3]2+ at the w|RTIL interface than those found at the w|DCE interface supported the previous observation of a greater distribution ratio in the aqueous–RTIL metal extraction than that in the aqueous–alkane processing. The kinetics of the reactions at the w|RTIL interface was slow. The stoichiometries at the w|DCE interface were confirmed using biphasic electrospray ionization mass spectrometry (BESI-MS) as well as direct injection of Sr2+ and CMPO mixture by means of a “shaking flask” experiment to conventional ESI-MS.