The photoionization mass spectra and photoionization efficiency curves of ArCO clusters are obtained with synchrotron radiation mass spectrometry. By comparison with absolute photoabsorption spectra of CO, the photoionization efficiency curve of ArCO clusters in an energy region from 13.9 to 14.6 eV reflects mainly the properties of Rydberg series converging to the X2+ (v+= 1, 2 and 3) of CO+, and these of n= 3 vibration sequence of the series converging to the A2 state of CO+. In the energy region from 14.6 to 15.75 eV, the curve reflects mainly the absorption property of CO, but its five strong peaks shift toward blue due to the interaction between Ar and CO. In an energy region from 15.75 to 15.80 eV, the curve reflects mainly the absorption properties of Ar and CO. At the same time, ionization energy of ArCO, and dissociation energies of ArCO and ArCO + are also calculated using the theory of quantum chemistry.