The microphase separation of a solvent-sensitive self-assembling block copolymer, poly(2-phenoxyethyl vinyl ether)-block-poly(2-methoxyethyl vinyl ether) (pPhOVE-pMOVE), in acetone/water mixture was investigated by small-angle neutron scattering (SANS). Acetone solutions of pPhOVE-pMOVE undergo tremendous viscosity thickening by adding small amount of water as a result of formation of spherical domains highly ordered in a body-centered cubic (bcc) lattice with interdomain entanglements of tethered chains. A contrast variation SANS allowed us to elucidate the architecture consisting of gathered pPhOVE domains with tethered pMOVE chains spread in the matrix. The structure evolution of this "water-induced microphase separation" was compared with that of heat-induced microphase separation. It was found that the former is an amplification process of concentration fluctuations like spinodal decomposition and the latter is analogous to a nucleation-and-growth mechanism.