GaAsN/GaAsN:H in-plane heterostructures have been investigated by secondary electron (SE) imaging in a field-emission scanning electron microscope. Adjacent GaAsN and GaAsN:H regions show a quite different SE image brightness due to the band gap energy difference between H-free and H-irradiated GaAsN. These findings provide a useful means to characterize the lateral diffusion of H and well support secondary ion mass spectroscopy results regarding the importance of a low hydrogenation temperature in order to obtain sharply defined in-plane heterostructures.