This paper presents a detailed flexible mathematical model for planar solid oxide fuel cells (SOFCs), which allows the simulation of transient performance characteristics. This mathematical model includes the incorporation of a thermal modelling into an electrochemical model with physico-chemical governing equations and a detailed multi-component gas diffusion mechanism, Dusty Gas Model (DGM). Spatial discretization can be applied up to quasi 3-D geometries and is resolved with the FDM (Finite Difference Method). The model is built and implemented in the commercially available modelling and simulations platform gPROMS™. The model was compared against existing models, the differences in results are identified and attributed to assumptions. Several transient operation case studies are examined such as load change and start-up: the results illustrate how important such a tool is for analyzing SOFC operation.