We describe a simple, low-cost, gas flow vapor sorption/desorption apparatus that enables "interval" or "differential experiments on supported polymer films, with control of small step changes in diffusant weight fraction, omega(1), to intervals as small as 0.1%. The apparatus permits accurate determination of the binary mutual diffusion coefficient D-12(omega(1)) for omega(1) up to at least 3% for "condensable" organic species that typically exhibit very sharp increases in D-12 with the diffusant's weight fraction. We report results for two homologous series of diffusants, n-alkyl acetates and alkyl benzenes, in monodisperse poly(methyl acrylate) (PMA). The Vrentas-Duda free volume theory adequately accounts for the strong concentration dependence of D-12 seen in all cases. Nearly constant values of the jump-unit size ratio, xi, for the homologous set of n-alkyl acetates in the range xi = 0.61-0.67 closely match those reported previously. For the alkyl benzenes, xi for benzene (xi = 0.78 +/- 0.04) was higher than those for the larger homologues toluene, ethylbenzene, and cument (xi = 0.58-0.60). The xi values for both homologous series are well-predicted by a free volume based estimate for eccentric diffusants, which jump as a single unit in a mixture whose components contribute different amounts of free volume per jump unit, according to their geometries.