The influence of framework flexibility on the adsorption and diffusion of methane in LTA zeolites was investigated by Monte Carlo and molecular dynamics simulations. In particular, we analyzed the framework flexibility of the pure silica structure (ITQ-29), the sodium form (LTA-4A), and the sodium/calcium form (LTA-5A). Simulations were performed at 500 K and over the full loading range. We found that the framework flexibility affects adsorption and diffusion of methane differently. The effect of flexibility on adsorption is small. However, the influence on diffusion seems to be much larger and strongly dependent on three factors: the density and type of the nonframework cations located in the LTA zeolite, the loading of methane in the structure, and, most importantly, the force field parameters used to model the framework.