Inductively-coupled plasmas in molecular, electronegative gases are widely used for plasma processing of surfaces, for instance in CMOS manufacture. The complexity of these systems is such that they can only be described by multi-physics models which describe both the plasma physics and the molecular collisional processes. However, there has been little rigorous validation of these models by comparison to quantitative measurements of particle densities over a wide range of parameter space. We have chosen to study the Cl2/O2 system partly because of the industrial process relevance but also because methods exist to measure the density of many of the particles present. Electron densities were measured by microwave hairpin resonator. Absolute Cl and O atom densities were determined by Two-photon Absorption Laser-Induced Fluorescence. We have constructed a new ultra-low noise broadband UV-visible absorption bench, which allows the measurement of the densities of ground state Cl2 molecules and ClxOy reaction products, as well as vibrationally excited states of Cl2 and O2.