FLUIDS confined in narrow pores can have properties that are distinctly different from those of bulk fluids(1,2). Most studies of fluids in pores have focused on simple fluids which can be modelled as near-spherical molecules. But molecular shape can also exert an influence on the fluid’s behaviour, particularly for large and/or complex molecules. The adsorption isotherms of alkanes in the zeolite silicalite provide an apparent example of this : the short-chain (C-1 to C-5) and long-chain (C-10) alkanes have simple isotherms(3-5) whereas for hexane and heptane the isotherms are kinked(4,6), suggesting that some kind of phase transition takes place. Here we present computer simulations of the adsorption of straight-chain hydrocarbons in silicalite, which suggest that this phase transition is of a type not reported previously, arising as a consequence of the interplay between the length of the zig-zag pores and the length of the alkanes. When these two are comparable, the molecules can ’freeze’ in a configuration that is commensurate with the pore structure, creating a kink in the isotherms. Such behaviour might be quite general for complex molecular fluids.