Steric and substituent effects can play large roles in influencing the outcomes of organic reactions. In this work, the use of ion-molecule reactions of dibenzo-16-crown-5 compounds (lariat ethers) by tandem mass spectrometry to probe the influence of the pendant groups on the selectivity of their gas-phase reactions was evaluated. Lariat ethers are macrocyclic ethers with pendant substituents that have been developed as new types of hosts for molecular recognition. Dimethyl ether (DME) was the reactant chosen because of its well characterized reactivity with various organic substrates possessing different functional groups. Only those dibenzo-16-crown-5 compounds with no or at most one substituent at the center carbon of the three-carbon bridge form the diagnostic [M + 13]+ product ion through a methylene substitution process. Dibenzo-16-crown-5 compounds with geminal substituents on the center carbon of the three-carbon bridge form the [M + 45]+ ion, but not the characteristic [M + 13]+ ion. Causative factors may be steric blocking of the reaction pathway by the geminal groups or a requirement for the presence of at least one hydrogen on the center carbon of the three-center bridge for formation of the [M + 13]+ ion. CAD, deuterium labelling, molecular orbital calculations and comparisons with model compounds provide additional information about the reaction pathways.