The complexation of tetrahydrofuran as well as 1,4-dioxane with water has been investigated by laser light scattering (LLS). The complexation of cycloethers with water molecules makes the diffusion of water clusters visible in LLS. The intensity-intensity time correlation function of a simple mixture of water and tetrahydrofuran (THF) or 1,4-dioxane has two diffusive relaxation modes. The fast mode with a diffusion coefficient in the range 2-8 x 10(-6) cm(2) s(-1) is related to mutual diffusion of the complexes formed between small water clusters and THF or 1,4-dioxane. The slow mode with a dynamic correlation length of 200-600 nm can be removed by filtration, indicating that the mixing of water and cycloethers is microscopically incomplete. Further, using a trace amount of spherical poly(N-isopropyl acryamide) microgels as probes, we indirectly investigated the water/THF interaction. Our results showed that PNIPAM swell in ether pure water or pure THF but fully collapse when [THF]:[water] = 0.05-0.15, in which the water/THF complexes are nonsolvent for PNIPAM. When [cycloether]: [water] > similar to0.16, water is not able to further complex with cycloethers added so that extra cycloethers exist as individual molecules to reswell the microgels, indicating that it is small water clusters, such as the pentamer and hexagon, that solvate PNIPAM.