Summary: Formation of modulated phase patterns can be modelized by a modified Cahn-Hilliard equation which includes a non-local term preventing the formation of macroscopic domains. Using stationary solutions of the original Cahn-Hilliard equation as analytical ansatzs, we compute the thermodynamically stable period of a 1D modulated phase pattern. We find that the period scales like the power \(-1/3\) of the strength of the long-range interaction.