A combination of experiments based on proton transfer reactions monitored in a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer and molecular orbital calculations up to the G2 and the CBS-Q levels demonstrates that the structure of the ions produced by electron ionization of cyclopropylamine, 1, and relaxed at thermal energy, possess the ionized 1-propene amine structure [CH3CHCHNH2]*+, 3*+. The experimental deprotonation enthalpy of ions 3*+ is equal to 915.3 ± 3.2 kJ mol−1. CBS-Q calculations are in good agreement with experiment. A value of 919-923 kJ mol−1 is calculated for the deprotonation enthalpy of ions 3*+; 298 K heat of formation values of 852 kJ mol−1 and 783 kJ mol−1 are predicted from G2 atomization energies for ions 2*+ and 3*+, respectively. The heat of formation of [CH2CHCHNH2]+ ions has been evaluated to 757.6 ± 5.7 kJ mol−1 from experiment and 755.8 kJ mol−1 from G2 atomization energy.