The complexes of protonated isomeric primary heptylamines (C7H15-NH2) with crown ethers have been studied by using electrospray ionization-collision induced dissociation-tandem mass spectrometry (ESI-CID-MS/MS). It has been found that besides the gas phase basicity, small structural changes in the amines may also lead to changes in the relative gas phase stabilities of the complexes. For example, 2-aminoheptane and 1,3-dimethylamylamine (the heptylamines that are on the list of the prohibited substances of the World Anti-Doping Agency) form complexes with DB24C8 of remarkably different relative gas phase stabilities. The H/D exchange in the complexes of protonated isomeric heptylamines with crown ethers may also lead to the visible differences in the relative gas phase stabilities.