Reaction of (CH3OH)-O-18 with O-16-containing isobutanol (2-methylpropan-1-ol) over strong acid Nafion-H and Amberlyst-H 35 resin catalysts gave the two distinctly labelled C-5 ethers 1-methoxy(O-16)-2-methylpropane (also designated as methyl isobutyl ether, O-16-MIBE) and 2-methoxy(O-18)-2-methylpropane (also designated as methyl tertiary-butyl ether, O-18-MTBE). Reaction of (CH3CH2OH)-O-18 with isobutanol gave the analogously labelled C-6 ethers, O-16-EIBE and O-18-ETBE. These results show that the isobutyl and tertiary-butyl ethers are formed from the alcohols by distinctly different mechanistic pathways, i.e, the former are produced by surface-catalyzed S(N)2 reactions that follow Langmuir-Hinshelwood kinetics involving competitive adsorption while the latter arise via carbenium or olefinic intermediates. There is no pathway for isomerization of the two ethers, MIBE and MTBE, under the reaction conditions employed.