The absorption spectra of the gases ethylene‐d0, d1, 1,1‐d2, cis‐d2, trans‐d2, d3, and d4 have been photographed using the first order of a 21‐ft vacuum spectrograph. Analyses of both the Rydberg system at 1750 A and the π*‐π system between 1500 and 2100 A have been extended. Band contour analysis has confirmed the assignment of the Rydberg system as 1B3u‐1Ag. The resulting rotational constants of the 42 level of the excited state of C2D4 lead to an inertial defect indicative of a nonplanar molecule. Assuming point group D2, we have combined the constants with Franck‐Condon calculations in a complete structure determination for the 42 level of the Rydberg state: rCC = 1.41 A, rCH = 1.08 A, [right parenthesis lessthan] HCH = 124.4° and an azimuthal angle of 37°. Linewidths in the successful simulation of the C2D4 0–0 band indicate a lifetime greater than 10−13 sec for molecules in the zero level of the Rydberg state. For the π*‐π system, all observed bands for all molecules studied have been assigned. Most...