IR spectra, thermodynamics and photochemistry of ethene, C₂H₄, physisorbed onto NaCl(100) cuboids of a sublimed polycrystalline film have been examined. The heat of adsorption of ethene onto the NaCl film ranges –10 ± 3 < Δ_adsH < –17 ± 3 kJ mol^–1 over a coverage range of 0.5 < Θ < 1.0. The vibrational spectrum of the adsorbed ethene resembles that of condensed phase and matrix isolated ethene where intensities and frequencies are only slightly modified from the gas phase. The ethene, adsorbed at less than a full monolayer on the crystallites, is then subjected to 184.9 nm light whereupon it photodissociates almost exclusively into acetylene and H₂. Further, it is not the ethene molecules, but the NaCl substrate which appears to be the main antenna of photon capture. We present evidence that suggests that the excitation is stored as self-trapped excitons (STEs) in the NaCl. This energy is then passed to the adsorbed ethene via triplet–triplet energy transfer. It is proposed that the acetylene is produced either through a triplet ethylidene or vinylidene intermediate.