Convergent-beam electron diffraction has been used to study strain modulation in GaAs/InGaAs superlattices (6% In and 2% In) in plan-view. Reflections from planes inclined to the interface of the superlattice are split with sidebands, the angular separation of which is related to the modulation periodicity. It is inferred from higher-order Laue zone lines that inclined planes on opposite sides of an interface in GaAs and InGaAs layers are rotated towards and away from the interface as a result of distortion due to coherent matching and local relaxation in regions where the GaAs substrate and buffer layer have been ion-milled away.