The proximity of the calcium/strontium binding site of the oxygen evolving complex (OEC) of photosystem II (PSII) to the paramagnetic Mn cluster is explored with Sr-87 three-pulse electron spin-echo envelope modulation (ESEEM) spectroscopy. CW-EPR spectra of Sr2+-substituted Ca2+-depleted PSII membranes show the modified g = 2 multiline EPR signal as previously reported. We performed three-pulse ESEEM on this modified multiline signal of the Mn cluster using natural abundance Sr and Sr-87, respectively. Three-pulse ESEEM of the natural abundance Sr sample exhibits no detectable modulation by the 7% abundance Sr-87. On the other hand, that of the Sr-87 enriched (93%) sample clearly reveals modulation arising from the I = 9/2 Sr-87 nucleus weakly magnetically coupled to the Mn cluster. Using a simple point dipole approximation for the electron spin, analysis of the Sr-87 ESEEM modulation depth via an analytic expression suggests a Mn-Ca (Sr) distance of 4.5 Angstrom. Simulation of three-pulse ESEEM with a numerical matrix diagonalization procedure gave good agreement with this analytical result. A more appropriate tetranuclear magnetic/structural model for the Mn cluster converts the 4.5 A point dipole distance to a 3.8-5.0 Angstrom range of distances. DFT calculations of Ca-43 and Sr-87 quadrupolar interactions on Ca (and Sr substituted) binding sites in various proteins suggest that the lack of the nuclear quadrupole induced splitting in the ESEEM spectrum of Sr-87 enriched PSII samples is related to a very high degree of symmetry of the ligands surrounding the Sr2+ ion in the substituted Ca site. Numerical simulations show that moderate Sr-87 quadrupolar couplings decrease the envelope modulation relative to the zero quadrupole case, and therefore we consider that the 3.8-5.0 A range obtained without quadrupolar coupling included in the simulation represents an upper limit to the actual manganese-calcium distance. This Sr-87 pulsed EPR spectroscopy provides independent direct evidence that the calcium/strontium binding site is close to the Mn cluster in the OEC of PSII.