In the comment on my Letter about the use of the thermodynamic cycles to calculate pK(a), da Silva and co-workers claim that my approach is incorrect due to an erroneous expression used for the chemical potential of water. In this reply, I have presented a rigorous statistical mechanics derivation of the chemical potential of liquid water, which shows that the expression for the chemical potential that I have used is correct. This result supports the previous conclusion that the thermodynamic cycle used by da Silva et al. leads to thermodynamically inconsistent equation for calculating the pK(a). (C) 2003 Published by Elsevier B.V.