Heretofore, it has been assumed that the Rydberg (3s) doublets appearing in the absorption spectrum of ethylene derive their intensity from dipole allowedness. This mechanism is consistent with a vibrational assignment of the spacing between members of the doublet pairs as methylene twisting,v′4, in the R state. Herein is presented an analysis based on the assumption that the Rydbergs derive intensity via vibronic stealing from the underlying V → N band. This mechanism presents, for the first time, an explanation for the doublet nature of the Rydbergs by offering two tenable vibrational modes through which intensity stealing can occur-v′8andv′4+v′10. This predicts a doublet spacing ofv′4+v′10-v′8, which provides excellent agreement between ground and excited state isotopic ratios (C2D4against C2H4), namely, 0.595 in N versus 0.594 in R. Predictions are made as to the positions of the dipole allowed bands, as well as the values ofv′2,v′4,v′6andv′10in the R states of C2H4and C2D4.