The structure of clusters formed in 1,4-dioxane-water binary solutions has been investigated at ambient temperature as a function of 1,4-dioxane concentration by X-ray diffraction for the corresponding solutions and by mass spectroscopy for liquid droplets formed in vacuum from the liquid mixtures by an adiabatic expansion method. The 2H spin-lattice relaxation times of D2O and 1,4-dioxane-d8 molecules in 1,4-dioxane-water binary solutions have also been measured at 30 °C over a whole range of 1,4-dioxane mole fraction. It has been found from the analysis of X-ray radial distribution functions that the number of hydrogen bonds per water and 1,4-dioxane oxygen atom decreases with increasing 1,4-dioxane mole fraction Xdio accompanied by two inflection points at Xdio = ∼-0.1 and ∼0.3: at Xdio ≤ 0.1 the hydrogen-bonded network of water is predominant in the binary solutions, at Xdio ≥ 0.3 the inherent structure of 1,4-dioxane is mostly observed, water molecules probably involved in the structure by hydrogen bonding, and at 0.15 ≤ Xdio ≤ 0.2 both structures of water and 1,4-dioxane are ruptured to form small binary clusters of one or two dioxane and several water molecules. The mass spectra have revealed that at Xdio = 0.01 water clusters Wn (W = water, n = 6 2- 43) are mostly formed, but with increasing Xdio to 0.4 the water clusters are reduced with evolving 1,4-dioxane clusters DmWn (D = 1,4-dioxane, m = 1 – 3, n = 1 – 16). The 2H spin-lattice relaxation data of D2O molecules in the mixtures showed that the rotation of water molecules is gradually retarded with increasing Xdio to −0.3, where the rotation is the slowest, and is then gradually accelerated with further increase in Xdio. The corresponding data of 1,4-dioxane-d8 molecules showed a similar tendency, but the slowest motion observed at Xdio = −0.2. The present microscopic cluster structure and dynamic properties of the mixtures are discussed in connection with the heat of mixing, viscosity, hydrophobic hydration, and clathrate hydrate.