In this work a mathematical model of the vegetable tanning process under different operating conditions has been developed and validated. The mathematical model describes the tanning process as a series process involving tannin diffusion into the leather and subsequent reaction with collagen. The external diffusion has been assumed to be very fast because of the high rotation rate of the drum. The change of hide thickness was also considered by analyzing the experimental data. The mathematical model well describes tanning processes for most of the operating conditions examined.