Perdeuterated ethyl iodide in solid parahydrogen is photolyzed at 4.4 K to find the formation of all deuterated ethylene, ethane, and ethyl radical and deuterium iodide. The temporal change in the intensity of the vibrational spectra upon UV irradiation reveals that the initial ethyl iodide exists in both monomeric and dimeric units. The monomeric unit is subjected to the following competitive reactions: C2D5I + h nu --> .C2D5 + .I and C2D5I + h nu --> CD2=CD2 + DI The ethylene produced thereby is loosely complexed with the counterpart DI. The dimeric unit undergoes the following one-photonic parallel reactions I and II: (I) (C2D5I)(2) + h nu --> 2 .C2D5 + I-2 to be followed by a gradual disproportionation, 2 .C2D5 --> CD2=CD2 + C2D6, which proceeds by quantum tunneling of a D atom between the radicals in the experimental time scale. The possible recombination of the two radicals to butane is not observed at all. (II) (C2D5I)(2) + h nu --> CD2=CD2 + C2D6 + h, which is a direct molecular process to give the same products as (I). The ethylene produced by both (I) and (II) tends to form complexes with C2D6 and with I-2. Prolonged irradiation induces the following secondary photolysis of the three primary photoproducts: .C2D5 + h nu --> CD2=CD2 + .D, DI + h nu --> .D + .I, and I-2 + h nu --> 2 .I.