AbstractThe influence on the excess scattering functionP(μ) of flutuations in the electron density ρ within a macromolecule is treated, to the approximation that the solvent is a structureless medium of constant electron density ρ0. The results forP(μ) and the apparent value of the mean square radiusRapp2, can be expressed as functions of the excess electron density Δρ:P(μ) =X(μ) + (Δρ)−1Y(μ) + (Δρ)−2Z(μ) andRapp2=Rx2+ (Δρ)−1Ry2+ (Δρ)−2Rz2, whereX(μ) andRx2depend only on the shape of the macromolecule, whileY(μ) andRy2as well asZ(μ) andRz2depend on the shape and the fluctuations in ρ. By varying the electron density of the solvent, the contributions of the shape and the internal structure of the macromolecule can be resolved. The quantitiesRx2,Ry2, andRz2are evaluated for seven models to illustrate the relative importance of these contributions