Store operated calcium entry (SOCE) is controlled by the ER-resident Stromal Interaction Molecule 1 (STIM1) which senses a decrease in [Ca2+]ER and opens Ca2+-selective channels of the Orai and TRPC families. The oligomerization of STIM1 initiated by a drop in [Ca2+]ER is the crucial step for SOCE activation, and by analogy SOCE termination is thought to be controlled by the de-oligomerization of STIM1 initiated by the restoration of physiological [Ca2+]ER levels. We quantify the [Ca2+]ER dependence of STIM1 oligo-/deoligo-merization during physiological stimulation of HeLa cells, using the D1ER to measure [Ca2+]ER and FRET between CFP-STIM1 and YFP-STIM1 to monitor STIM1 oligo-/deoligo-merization. We found that the kinetics of STIM1 oligomerization follow the kinetics of store depletion independently of the type of stimulation used. We also showed that, in addition to store refilling, a local increase in the cytosolic Ca2+ concentration is required to initiate STIM1 de-oligomerization.