Using physical concepts, an integrated transformation model to describe the kinetics of ferrite and bainite formation from work-hardened austenite has been developed for a Mo-TRIP steel. The ferrite sub-model assumes a mixed-mode kinetics under paraequilibrium condition and accounts explicitly for the effect of alloying elements by considering their interaction with the moving ferrite-austenite interface. To predict the onset of bainite formation, which corresponds to the cessation of ferrite reaction along a given cooling path, a criterion based on a critical driving pressure is formulated. Regarding the kinetics of the subsequent bainite reaction, the proposed model adopts the Zener-Hillert diffusional approach. The proposed integrated model has been employed to describe the continuous cooling transformation kinetics for a 0.19C-1.5Mn-1.6Si- 0.2Mo (wt%) TRIP steel that had previously been subjected to a systematic experimental study. The predictive capabilities of the model and the challenges for further model improvements are delineated.