Molecular dynamics simulation of the modified strong dipole-proton coupling model is used to show that ferroelectric phase transition temperature, T-c, in KH2PO4 (KDP) and KD2PO4 (DKDP) linearly depends on hydrogen bond length, RO-O, and separation between peaks in proton (deuteron) thermal distribution in the paraelectric phase, delta. Separation between lines for KDP and DKDP is larger in the T-c-RO-O dependence than in the T-c-delta dependence indicating equal effect of different RO-O and different proton and deuteron masses on the whole change of T-c between KDP and DKDP and establishing delta as a more important determinant of T-c in these crystals. (C) 2004 Elsevier B.V. All rights reserved.