The reaction mechanism of etherification of beta-citronellene with ethanol in liquid phase over acid zeolite beta is revealed by in situ solid-state C-13 NMR spectroscopy. Comparison of C-13 Hahn-echo and H-1-C-13 cross-polarization NMR characteristics is used to discriminate between molecules freely moving in liquid phase outside the zeolite and molecules adsorbed inside zeolite pores and in pore mouths. In the absence of ethanol, beta-citronellene molecules enter zeolite pores and react to isomers. In the presence of ethanol, the concentration of beta-citronellene inside zeolite pores is very low because of preferential adsorption of ethanol. The etherification reaction proceeds by adsorption of beta-citronellene molecule from the external liquid phase in a pore opening where it reacts with ethanol from inside the pore. By competitive adsorption, ethanol prevents the undesired side reaction of beta-citronellerie isomerization inside zeolite pores. beta-citronellene etherification on zeolite beta is suppressed by bulky base molecules (2,4,6-collidine and 2,6-ditertiarybutylpyridine) that do not enter the zeolite pores confirming the involvement of easily accessible acid sites in pore openings. The use of in situ solid-state NMR to probe the transition from intracrystalline catalysis to pore mouth catalysis depending on reaction conditions is demonstrated for the first time, The study further highlights, the potential of this NMR. approach for investigations of adsorption of multicomponent mixtures in general.