A diphthalocyanine disulfide (Pc) molecule has been deposited as a monolayer on gold-coated substrates through the process of self-assembly. To establish the molecular orientation of the Pc molecule on the gold surface the two complementary techniques of transmission IR and reflection absorption IR (RAIR) spectroscopies were used. The appearance of IR absorption bands associated with the Pc nucleus in the transmission spectrum, and their absence in the RAIR spectrum, suggests that the Pc self-assembled monolayer (SAM) is orientated with the macrocycle parallel to the metal surface. The Pc SAM has been used in conjunction with surface plasmon resonance (SPR) to establish the utility of combining these techniques for optical gas sensing. The SPR reflectivity curves for the gold substrate and the Pc SAM on the gold substrate have been obtained. On exposure of the Pc SAM to the environmentally important NO₂ gas, changes of the reflectivity signal were obtained in proportion to the concentration of the analyte gas. The results obtained show that the monolayer deposition technique of self-assembly is an ideal method for the production of chemically sensitive substrates which can be combined with surface plasmon resonance for the optical sensing of gaseous species.