The microdomain structure of segmented polyether polyurethaneureas is investigated by means of positron annihilation lifetime spectroscopy, small-angle X-ray scattering and differential scanning calorimetry. The experimental results show that the decrease in the domain volume and free volume results from the increase in the hard segment (polyuretha-neurea segment) contents as the number-average molecular weight $\overline{M_n}$ of the soft segments (polyethylene glycol segments) is the same, and that the increase in domain volume and free volume result from the increase in the $\overline{M_n}$ of the soft segments when the hard segment content is the same or nearly the same. These results demonstrate that positron annihilation lifetime spectroscopy is a sensitive technique to probe the microdomain structure in polymers.