This paper discusses a phenomenological kinetic model which describes the conversion of complex non-homogeneous systems. The model embodies the concept of a severity parameter as a descriptor of the extent of reaction of the complex system. Temperature, time and catalyst concentration are simply and conveniently regrouped in the definition of the severity parameter. The link between our model and formal kinetics can be made via a time-dependent rate constant. Such dependency expresses the concept that in a complex non-homogeneous system the chemical acts leading to reaction are first order processes constrained either by microstructural effects, diffusional limitations, geometric discontinuities, or in generic terms by fractal considerations. The observable effect is the change in reactivity with extent (or time) of reaction. Our approach permits to model such systems via a set of four key markers characteristic of the overall reaction. The development of the model, the significance of the markers and their application to the hydrolysis of lignocellulosic biomass as a case study are presented and discussed.