The structure and dynamics of poly(ethylene oxide) (MW 400, PEO400) intercalated in the galleries of a layered fluoromica clay were investigated by spin-label electron spin resonance (ESR), XRD, DSC, and TGA. PEO400 was end-labeled by attachment of a nitroxide radical and intercalated in 0.78 nm clay galleries. The ESR spectra of intercalated PEO400 showed two motional components, fast and slow, which were attributed to polymer segments weakly and strongly interacting with the clay interface, respectively, while a single spectral component was measured for bulk PEO400. The rotational diffusion coefficient for the polymer chain segment, R-S, was determined by simulating the temperature variation of the ESR spectra, based on the "macroscopic order with microscopic disorder" (MOMD) model. The effect of the molecular weight was assessed by comparison with the previous study of PEO4000 (PEO4K [Miwa, Y.; et al. Macromolecules 2008, 41, 4701-4708]. The values of log R-S for bulk PEO400 and PEO4K at 330 K were 8.8 and 7.4, respectively; the higher mobility of bulk PEO400 is considered to be a result of the higher concentration of chain ends, which leads to their enhanced mobility through cooperative motion. In contrast to bulk PEO, the mobility of the spin-labeled intercalated PEO was not enhanced, indicating that the cooperative segmental motion of PEO in the galleries is reduced. The smaller A(zz) value (the N-14 hyperfine splitting along the z axis of the 2p(z) orbital of the unpaired electron) for intercalate PEO400 compared to the bulk polymer was taken as an indication that hydrogen bonding between the nitroxide labels and the hydroxyl chain ends of PEO400 in the galleries is hindered. From these results, it was concluded that interactions between PEO segments are negligible and cooperative segmental motions are reduced in the galleries.