The interaction between CaCl2 and double hydrophilic block copolymer: poly(ethylene oxide)(45)-block-poly(acrylic acid)(70), PEO45-b-PAA(70), was investigated. At a stoichiometric ratio of Ca2+:COO- = 0.5, Ca2+ ions were bound to COO- groups on PAA segments via electrostatic interaction. Small particles of 4-8 nm in diameter were observed, suggesting the formation of coil-like polymeric globule induced by charge neutralization. At Ca2+:COO- >= 2.5, monodispersed aggregates of average hydrodynamic diameter of 52.0 +/- 7.4 nm were produced. The ISE, ITC, surface tension and fluorescence spectroscopic data confirmed that the formation of these aggregates is not the result of interaction between excess Ca2+ ions and the polymer, but rather it is due to changes in the water activity that triggers. the structural rearrangement of Ca2+/PEO45-b-PAA(70) complex.