The first observations of vapour phase fluorescent exciplexes, formed between photo-excited substituted aromatic molecules and one of a variety of ground state species, are reported. From fluorescence decay and steady-state quenching results, the rate constants for the various formation and decay processes of one such exciplex, that formed between 1,4 Dicyanonaphthalene (1,4 DCN) and 2,5 Dimethyl 2,4 Hexadiene (DMHD) were extracted. The thermodynamic properties of this exciplex were also obtained from the temperature dependence of these rate constants. The values of these thermodynamic properties were compared with those found by other workers for similar systems in the solution phase. A CNDO/S-CI molecular orbital method was used to predict the electronic structures of some substituted benzene molecules. Using such a method the energies of the excited singlet and triplet energies for such species were calculated. These values were then used in conjunction with a group theoretical analysis to suggest assignments for some of the electronic bands in various vapour phase triplet-triplet absorption experiments. A novel sampling technique for the detection of atmospheric pollutants has also been described.