Large‐angle Rayleigh scattering of light provides a tool for studying fluctuations in dilute gases where the wavelength is comparable to the mean free path. Interpretation of such an experiment requires a molecular calculation of the spectrum of the density fluctuation. The initial value problem can be formulated using the linearized Boltzmann equation. Solutions are obtained for two types of repulsive intermolecular potential, Maxwell molecule and rigid sphere, and the results are used in a quantitative analysis of high‐resolution measurement. The problem of studying molecular gases in a similar way is also discussed.