The Fourier transform pulsed field gradient NMR technique has been applied to study the diffusion of coadsorbed molecules in large zeolite crystals. The two systems studied were (i) methane/ethylene in NaY and (ii) ethane/ethylene in silicalite. The values obtained for these systems were compared with single-component diffusivities. In NaY, the self diffusivity of methane, though higher than ethylene by an order of magnitude, was reduced in comparison to its single component value by a factor of 2 (at the same total sorbate loading). In silicalite, the diffusivity of ethane was comparable to coadsorbed ethylene, having fallen 2 orders of magnitude below the single-component ethane diffusivity in silicalite (at the same total sorbate loading). This result confirms the expectation, due to Pfeifer et al. [J. Am. Chem. Soc. 1991, 113, 4812], that slowly diffusing sorbates act to reduce the diffusivity of otherwise very mobile sorbates. In NaY though, ethylene does not block diffusion as effectively as in silicalite, probably because (on account of the larger pore size and higher connectivity in NaY) methane can find paths around the immobile ethylene.