More than 40 different molecules have been observed in Interstellar space within the Galaxy. Many of these molecules are seen near hot OB-type stars whose far-ultraviolet emission is intense. Laboratory spectroscopy of such molecules will lead to a better understanding of their astrophysical behaviour, in particular Celestial Masers. This thesis is preoccupied with the spectroscopy of H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O, D$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O, OH, CO, and C$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ in the region 1200$\backslash AA$-400$\backslash AA$ and absolute absorption coefficients are derived for CO and C$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ using the Thomas-Reiche-Kuhn Sum Rule. A possible Rydberg series of OH was observed, converging to a first ionisation potential of 13.36 eV and a likely inner shell two-electron resonance detected in the ionisation continuum of D$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$O at 415$\backslash AA$. Both a synchrotron and a laser produced plasma continuum source were used for absorption measurements and the advantages of each source were compared. Experiments were undertaken to examine the radiation properties of laser produced plasmas with a view to their general use as far-ultraviolet continuum sources and a computer program was written to predict their net photon emission by Bremsstrahlung processes, with some success. Optoacoustic spectroscopy was considered as a possible technique in the far-ultraviolet with a synchrotron source; its theory was extended to include ionisation and dissociation of molecules. However, optoacoustic experiments attempted were unsuccessful because of insufficient light intensity and excessive background noise near the synchrotron.