An oscillatory diffuse scattering from alpha-RbAg4I5 was observed by neutron diffraction measurement over the range sin theta/lambda of 0.1 to 0.8 Angstrom(-1) at room temperature. Numerical calculations of the diffuse background have been made based on the crystal structure of alpha-RbAg4I5 including the effects of the correlation between the thermal displacements of atoms. The nearest-neighbour silver and iodine correlations were found to be necessary to explain the oscillatory diffuse scattering.