We have studied Ca+(C2H4) by photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer over the spectral range 440-790 nm. Ca+ is the only photofragment observed. We find four absorption bands of the complex and assign them to metal-centered transitions correlating with excitation of Ca+(3d and 4p). Spectral assignment is supported by ab initio electronic structure calculations of the complex and isotope substitution experiments. Calculations find a weakly bound ground state equilibrium structure with C-2V pi-bonding geometry and a dissociation energy of D-e"=0.506 eV. Theoretical and experimental results show the 4p pi(2 B-2(2) & 2 B-2(1)) excited states to be relatively weakly bound at long range. Spectral analysis gives vibrational constants for the Ca+--C2H4 intermolecular a(1)-stretch in the 1 (2)A(1), 2 B-2(1), and 2 B-2(2) states, and for the CH2-CH2 a(1)-wag and the HCH a(1)-bend in 2 B-2(2). The results offer an interesting comparison with previous studies of similar weakly bound bimolecular complexes of light metal ions with alkene or alkane hydrocarbons. (C) 2000 American Institute of Physics. [S0021-9606(00)00215-4].