In this case study, we apply a recently developed method to systematically predict the linear dependencies in concentration profiles and identify minimum requirements to enable optimisation of rate constants and pure component spectra via direct multivariate kinetic hard-modelling of spectroscopic data. This systematic method was applied to the rank-deficient acid catalysed reaction of benzophenone with phenylhydrazine in THF. Various experimental conditions (different dosing and initial concentrations) and data treatments (defining uncoloured species, including known component spectra into the analysis) were considered. For all these conditions, the kinetic mechanism of this condensation reaction was successfully validated by the agreement between fitted and independently measured mid-IR and UV–vis pure component spectra and by the highly reproducible fitted rate constants. This case study particularly demonstrated the value of the direct spectral fitting as a tool for the validation of rank-deficient kinetic mechanisms, as inherent contributions within the fitted component spectra, due to the definition of uncoloured species, can be systematically addressed.