The microstructure and dynamic mechanical properties of segmented polyurethaneurea (SPUU) in concentrated dimethyl acetamide (DMAc) solutions have been investigated by means of small-angle X-ray scattering (SAXS) and viscoelastic measurements, respectively. Kratky plots of SAXS intensity functions indicated the following : (1) Formation of hard segment domains introduced mainly by hydrogen bondings is promoted with increasing ratio of isocyanate to hydroxyl groups ([NCO]/[OH]) and polymer concentration, (2) The degree of completion of the hard segment domains increases with temperature up to 55 degrees C and then gradually decreases by further increase of temperature. An analogy was found between [NCO]/[OH] dependence of the dynamic storage G’(omega) and loss shear moduli G "(omega) for SPUU in DMAc and time dependence of those for a chemically gelling system. G’(omega) and G "(omega) indicated that the properties of SPUU solutions change from sol-like to gel-like with increasing [NCO]/[OH]. By varying [NCO]/[OH], a relationship of G’(omega) approximate to G "(omega) approximate to omega(k) was observed around [NCO]/[OH] = 1.6. The exponent k is evaluated to be 0.67, which was similar to the literature value obtained at the gelation point. This corresponds to the ’gelation threshold’ of the chemical gelation process.