In zeolites and other rigid solid-state oxides, substrates whose sizes exceed the pore dimensions of the material are rigorously excluded. Now, using a porous 3 nm diameter capsule-like oxomolybdate complex [{(Mo6O21)-O-VI(H2O)(6)}(12){((Mo2O4)-O-V)(30)(OAc)(21)(H2O)(18})](33-) as a water-soluble analogue of solid-state oxides (e.g., as a soluble analogue of 3 A molecular sieves), we show that carboxylates (RCO2-) can negotiate passage through flexible Mo9O9 pores in the surface of the capsule and that the rates follow the general trend R = 1 degrees >> 2 degrees > 3 degrees >> phenyl (no reaction). Surprisingly, the branched alkanes (R = iso-Pr and tert-Bu) enter the capsule even though they are larger than the crystallographic dimensions of the Mo9O9, pores. Four independent lines of spectroscopic and kinetic evidence demonstrate that these organic guests enter the interior of the capsule through its Mo9O9 apertures and that no irreversible changes in the metal oxide framework are involved. This unexpected phenomenon likely reflects the greater flexibility of molecular versus solid-state structures and represents a sharp departure from traditional models for diffusion through porous solid-state (rigid) oxides.