The adsorption of n-alkanes (n-pentane to n-nonane) on the mesoporous MIL-100(Cr) and the MIL-101(Cr) rigid solids has been performed with a view of evaluating the influence of the alkyl chain length in terms of sorption capacity and sorbate/sorbent affinity. MIL-100(Cr) exhibits at first sight a classical type I behaviour towards the sorption of n-alkanes. These sorption isotherms have proven to be fully reversible. A direct relationship between amounts of alkane adsorbed at saturation and number of carbon atoms present in the adsorbed species has been found. At very low pressure (p/p 0 <0.025), sub-steps are present in the isotherm curve, due to the presence of two sets of cages (≈24Å and 29Å) and windows (≈5Å and 9Å). In the case of MIL-101(Cr), the sub-steps are shifted to higher partial pressures (p/p 0 <0.10) due to the larger size of the cages (≈27Å and 34Å) and windows (≈12Å and 16Å). For the same apolar sorbates, this outlines the effect of pore dimensions of the two MOFs, which are larger in the case of the MIL-101(Cr) structure, and gives rise to important differences in terms of affinity or sorption mechanisms. This is confirmed by the determination of the enthalpy of adsorption of n-hexane or n-heptane much lower in the case of MIL-100 as compared to MIL-101, at low coverage, −65kJmol −1 and −30kJmol −1 , respectively. The kinetics of the sorption process has also been discussed by comparing the equilibration times of the different sorbates during their adsorption. n-Nonane exhibited much longer equilibration times as to compared to shorter n-alkanes.