We present an ab initio direct dynamics study of the primary and solvent kinetic isotopes effects (KIEs) for the water-assisted tautomerization in the formamidine-water complex. These calculations are based on a variational transition state theory plus multidimensional semiclassical tunneling corrections with potential energy information calculated at the MP2 level of theory using the 6-31G(d,p) basis set. We found thar both the primary and solvent KIEs are large at low temperatures and are due not only to tunneling but also to quantum effects in vibrational motions. The primary KIEs were larger than the solvent KIEs. This results from differences due to the effect of deuteration on vibrational modes. Bending modes show significant inverse KIEs, while those of the reactive OH and NH vibrations show both inverse and normal effects. Such effects can be explained by examining changes in the zero-point energies of these modes. The adherence of the HH/HD/DD rates to the rule of the geometric mean is also examined.