The thermomechanical treatment of the TRIP (TRansformation Induced Plasticity) steels involve intercritical annealing followed by cooling to the austempering temperature. The composition of TRIP steels is adjusted in order to optimize the resulting multi-phase microstructure. Most commercial TRIP steels have compositions based on significant silicon additions. Silicon however causes the formation of adherent scale during rolling and the resulting surface is usually not adequate to use in exposed parts in automotive applications. Significant work is underway to evaluate the possibility of replacing silicon with other elements, such as aluminum, for instance. The correct understanding of the processes occurring during all stages of the heat treatment of these steels is a critical part of this evaluation. In this work, attempts to describe the transformations during the intercritical annealing and the cooling to the austempering temperature using computational thermodynamics (Thermo-calc) and diffusion simulation (DICTRA) are described. The results of calculations are compared with available experimental data and the limitations and the potential of using this approach in the alloy design of TRIP steels are discussed.