The ionization energies of MHP (CH 3 OOH) and EHP (CH 3 CH 2 OOH) have been determined by HeI photoelectron spectroscopy (PES) measurement and both Gaussian-2 (G2) calculation and Hartree-Fock (HF) method on the basis of Koopmans theorem at 6-311＋G* basis set level for the first time. The assignment and characterization of PE spectra of MHP and EHP were also supported by the G2 and HF calculations. The first ionization energies of MHP and EHP are 9.87 and 9.65 eV, respectively. Higher solubility of EHP in the atmosphere was attributed to their lower ionization energy values.