A new device/technique, Gas Reactor And Solid Sample Hopper designated GRASSHopper II, is described as an alternative to magic-angle spinning (MAS) for in situ NMR experiments on reacting heterogeneous systems. It is capable of supporting NMR studies of heterogeneous catalysis in the batch mode, in the flow reactor mode and for measurements of chemical kinetics. This new approach is based on the technique of magic-angle hopping (MAH). Characteristics, advantages, limitations and potential areas of application are described. Preliminary results are shown on the formation, subsequent hydrolysis and then regeneration (via dehydration) of (C6H5)(3)C+ carbocations in zeolite HY cavities, and on the gas-solid polymerization of ethylene on a Cp2ZrCl2/MAO/SiO2 ＇supported＇ catalyst.