The infrar-red spectra of gaseous HC=CCHO, DC=CCHO, HC=CCDO and the Raman spectrum of liquid HC=CCHO has been measured and analysed to give the normal vibrational frequencies of the molecule in the electronic ground state. The electronic band spectrum of propynal with origin at 4144 A has been photographed under low resolution in both emission and absorption and analysed in terms of the vibrational frequencies associated with the combining electronic states. The origin band for both HC=CCHO and DC=CCHO has been photographed under high resolution and a partial rotational analysis is presented for this band. The transition responsible for the electronic spectrum has been identified as being of n ->3pi*, 3A"<- 1A' type, and the observed spectrum confirms the theoretical predictions as to the structure that results from a transition of this type. / Thesis / Doctor of Philosophy (PhD)