Measurements are reported of the osmotic swelling and elastic properties of cross-linked polyisoprene gels swollen in toluene, as a function of the polymer volume fraction φ. The swelling pressure and shear modulus results may be combined to yield the Flory-Huggins interaction parameter χ for the cross-linked state of this system. It is found that χ can be expressed as χ = χ0 + χ1φ + χ2φ2, where χ0 = 0.427, χ1 = 0.000, and χ2 = 0.112. Small-angle neutron scattering (SANS) measurements are also reported for the same gels. The scattering intensity associated with the spontaneous thermodynamic concentration fluctuations in the gels is compared with that computed from the osmotic data. The agreement between the observed neutron intensities and the calculated values is good. The SANS measurements reveal two additional components to the scattered intensity: one due to the local elastic constraints of the cross-links and the other being caused by dense clusters that show no appreciable deformation when subjected to an external elastic strain field.