The complexation reactions between strontium (Sr2+) and octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) were studied at the aqueous vertical bar 1,2-dichloroethane (w vertical bar DCE) and aqueous vertical bar room-temperature ionic liquid (w vertical bar RTIL) microinterfaces, in order to understand its extraction in reprocessing spent nuclear fuels, remediation of environmental contamination, and potential radiological isotope feed stock for Y-90 from its isotope Sr-90 in fission byproducts. The stoichiometry (or metal to ligand ratios) and overall complexation constant (beta) for these reactions at these two interfaces are described herein. Two stoichiometries at the w vertical bar DCE interface were discovered, that is, Sr(CMPO)(2) and Sr(CMPO)(3) with beta values of 4.5 x 10(19) and 5.5 x 10(25), respectively. Only one stoichiometry was observed at the w vertical bar RTIL interface: Sr(CMPO)(3) with beta equal to 1.5 x 10(34). The larger complexation constant for Sr(CMPO)(3) at the w vertical bar RTIL interface than those found at the w vertical bar 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 vertical bar RTIL interface was slow. The stoichiometries at the w vertical bar 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.