The morphological changes of phosphonate polypeptoid electrolyte membranes, poly-N-(2-ethyl)hexylglycine-block-poly-N-phosphonomethylglycine (pNeh(m)-b-pNpm(n)), in hydrated and dry states were characterized by cryogenic transmission electron microscopy (cryo-TEM) and cryogenic electron tomography (cryo-ET). The analysis of 3D tomograms revealed that the pNeh(9)-b-pNpm(9) thin films absorbed a large amount of water, resulting in the formation of membranes that were nearly flat and giant multicompartment vesicles dispersed in the water phase. A simple lamellar phase appeared when the films were dried. In contrast, pNeh(18)-b-pNpm(18) thin films absorbed little water and formed small highly curved unilamellar and multilamellar vesicles. Water was located mainly outside the closely-packed vesicles. When water was removed by drying, the walls of adjacent vesicles collapsed to form honeycomb-like capsules. The changes in domain size reflected changes in chain conformations. The pNpm(9) blocks were saturated by water and fully extended, while pNpm(18) blocks were neither saturated by water nor fully extended. In addition, the thicknesses of hydrophobic blocks in the hydrated films of both pNeh(9)-b-pNpm(9) and pNeh(18)-b-pNpm(18) were smaller than those in the dry films, reflecting an increase of the average distance between the neighboring junctions of polypeptoid molecules.