For the first time, FTIR-ATR spectroscopy was used to study the water uptake and its diffusion in ion-selective membranes (ISMS) based on poly(acrylates) (PAS) and Silicone rubber (SR), which are emerging materials for the fabrication of ISMs for ultratrace analysis. Three different types of PA membranes were studied, consisting of copolymers of methyl methacrylate with n-butyl acrytate, decyl methacrylate, or isodecyl acrylate. Numerical simulations with the finite difference method showed that in most cases the water uptake of the PA and SR membranes could be described with a model consisting of two diffusion coefficients. The diffusion coefficients of the PA membranes were approximately 1 order of magnitude lower than those of plasticized poly(vinyl chloride) (PVC)-based ISMs and only slightly influenced by the membrane matrix composition. However, the simulations indicated that during longer contact times, the water uptake of PA membranes was considerably higher than that for plasticized PVC membranes. Although the diffusion coefficients of the SR and plasticized PVC membranes were similar, the SR membranes had the lowest water uptake of all membranes. This can be beneficial in preventing the formation of detrimental water layers in all-solid-state ionselective electrodes. With FTLR-ATR, one can monitor the accumulation of different forms of water, i.e., monomeric, dimeric, clustered, and bulk water.