Understanding multicomponent diffusion in polymers is important for many applications, including membrane-based separations and fuel cells. Although there are a number of publications documenting multicomponent diffusion models in polymers, there is limited in situ experimental evidence of multicomponent diffusion in polymers. In this work, a classic problem in fuel cells has been investigated. Specifically, for the direct methanol fuel cell (DMFC), methanol crossover (high methanol flux) is a critical concern as it deters both fuel efficiency and power output by interfering with the cathode reduction reaction. Therefore, a fundamental understanding of multicomponent diffusion of methanol and water in Nafion is desired. In this study, the diffusion of methanol/water mixtures in Nafion was measured using time-resolved Fourier-transform infrared, attenuated total reflectance (FTIR-ATR) spectroscopy. This technique not only provides in situ molecular-level contrast between diffusants and polymers in real time, but also can measure chemical interactions between diffusants and polymers through shifts in the infrared spectra. The multicomponent diffusion data from this work has been compared to classic multicomponent diffusion models using the generalized Stefan-Maxwell and Onsager formalisms derived from irreversible thermodynamics. More specifically, “main-term” and “cross-term diffusion coefficients have been regressed from the data. These results will be discussed in relation to model constraints presented in the literature, additional models that account for diffusant-diffusant and diffusant-polymer interactions, multicomponent diffusion effects (the effect of one diffusant's concentration gradient on the flux of another diffusant), and the overall impact of these results on methanol fuel cells.