High pressure neutron diffraction data on deuterated propane at four supercritical states and on a mixture of 39.93 mole % deuterated propane and 60.07 mole % deuterated methane at two supercritical states are presented. In the case of propane the density could be varied by a factor larger than 2. The molecular structures of deuterated propane and deuterated methane were determined. The density dependence of the intermolecular structure is used in order to test various united-atoms potential for propane in Monte Carlo simulations. The best agreement with the neutron diffraction data is obtained with the parameters from the AUA4 (anisotropic united-atoms) force field recently proposed.