Recently, the transition point between the Harrison Type-A and Type-B kinetics regimes as well as the emerging intermediate AB transition regime have been analysed in detail by making use of Lattice Monte Carlo (LMC) simulations of tracer depth concentration profiles as a function of diffusion time and distance between grain boundaries e.g. [1-3]. In the present study, we analyse Harrison Type-B to Type-C kinetics regimes in the transient grain boundary diffusion problem using the parallel slabs model and LMC numerical simulation. The transition point where the Harrison Type-B kinetics regime last occurs (transition point between the Harrison Type-B kinetics and the Type-BC kinetics) is estimated at (= 0.5δ(Dlt)-1/2) = 0.1. The Harrison Type-C grain boundary diffusion kinetics regime is also analysed using LMC simulated concentration depth profiles. The transition point where the Harrison Type-C kinetics regime first occurs (transition point between the Type-BC kinetics and the Harrison Type-C kinetics) is estimated at = 5.0. Therefore an intermediate Type-BC regime can be expected to occur between 0.1 < < 5.0. Preliminary results for the cubic grain model show that the interval for the intermediate Type-BC regime is somewhat narrower for this model and occurs at 0.5 < < 5.0.