The effect of pressure to 65 kbar has been measured on the conformation of two oligo (in-phenylene ethynylene) foldamers with 16 and 24 monomer units containing a piperazine or terpene derivative as a guest. The piperazine is cylindrical. whereas the terpene is roughly spherical. The results are compared with previously described studies on oligomers with no guest. In all cases, the oligomer tends to transform from the coiled (helical) conformation to an open conformation with increasing pressure. Equilibrium constants (K) were extracted from the luminescence spectra. The difference in partial molar volume Delta(V) over bar' between the conformations was obtained from the pressure derivative of K. For the oligomer with no guest, Delta(V) over bar' decreases rapidly with increasing pressure which shows that the helical conformation is more compressible. With the guest present, the decrease in Delta(V) over bar' is much smaller, and in some cases, there is an increase over some range of pressure. The presence of the guest drastically reduces the compressibility of the helical form, presumably because the guest fills the void space created by the helical cavity. A detailed comparison is made between the two oligomers with and without the guests present.