The energetics and dissociation dynamics of the (C2H4)+3 complexes have been studied by photoionization of neutral van der Waals ethylene trimers. The major product channels identified in the unimolecular decomposition of (C2H4)+3 in the total energy [neutral (C2H4)3 heat of formation plus excitation energy] range of ∼260–336 kcal/mol are C3H+6 +C3H6, C3H+7 +C3H5, C4H+7 +C2H5 (or C2H4+H), C4H+8 [or (C2H4)+2 ]+C2H4, C5H+9 +CH3, and C6H+11+H. The fact that these product channels are similar to those observed in the unimolecular decompositions of (C3H6)+2 and (c‐C3H6)+2 is consistent with the interpretation that the (C2H4)+3 , (C3H6)+2, and (c‐C3H6)+2 loose complexes rearrange to similar stable C6H+12 ions prior to fragmenting. The ionizationenergies (IE) of (C2H3)3 and (C2H4)4 are determined to be 9.465±0.036 (1310±5 Å) and 9.287±0.034 eV (1335±5 Å), respectively. Using the known IE’s of (C2H4)n, n=2, 3, and 4, and the estimated binding energies of (C2H4)2 ⋅ C2H4 and (C2H4)3 ⋅ C2H4, the bond dissociation energies for (C2H4)+2 ⋅ C2H4 and (C2H4)+3 ⋅ C2H4 are deduced to be 9.2±1 and 4.6±1 kcal/mol, respectively.