Kim and Kim [Phys. Rev. B 59, 13 509 (1999)] have studied the frequency dependence of the complex permittivity of $\small\sf KH_2PO_4$ and $\small\sf RbH_2PO_4$ in the ferroelectric phase. In the low-frequency regime ( $f<10$ kHz) they found a dispersion consisting of at least three distinctive components. Considering the temperature dependence of those relaxational modes they claim that each of them is associated with domain wall motion, and not with a heat diffusion central peak (HDCP). The aim of this Comment is twofold: First we want to demonstrate the importance and influence of the ac measuring field on the dielectric properties, in particular in the temperature range $T<T_f=100 K<T_C$ of domain freezing. Second, by investigating the dielectric nonlinearity in the limit of \small ac fields we exclude HDCP contributions in the low-frequency part of the spectrum.