Inductively-coupled radiofrequency plasmas in molecular, electronegative gases are widely used for plasma processing of surfaces, and in particular for the etching of thin films. Such plasma processes have allowed the information revolution since they are essential for the manufacture of CMOS integrated circuits. The complexity of these systems is such that they can only be described by multi-physics models which simultaneously describe a) the dynamics of charged particles and electric fields, b) the elastic and inelastic collisional pro- cesses between charged and neutral (molecular) species, and c) plasma-surface interactions. However, these models have not previously been rigorously tested against comprehensive quantitative measurements of transient particle densities over a wide range of parameter space. We have studed the Cl2/O2 system because of its industrial process relevance but also because methods exist to measure the density of most of the transient species present. A microwave hairpin resonator is used to measure the electron density, and (combined with laser photo-detachment) the negative ion density. Absolute Cl and O atom densities are determined by Two-photon Absorption Laser-Induced Fluorescence, combined with novel calibration schemes. A new ultra-low noise broadband UV-visible absorption bench allows measurement of the densities of ground state Cl2 molecules, ClxOy reaction products and vibrationally excited states of Cl2 and O2. These measurements were compared to a state of the art 2-dimensional hybrid fluid model, showing considerable discrepancies in both the absolute values and trends with gas pressure.The models can be improved significantly by paying attention to gas heating mechanisms, surface processes (atom recombination and ther- mal accommodation) and by improved cross-sections for processes including electron-impact dissociation of molecules and vibrational excitation.