The temperature and frequency dependent dielectric susceptibilities of $\sf\small KH_2AsO_4$ and $\sf\small KD_2AsO_4$ were studied in the ferroelectric phase. A relaxational dispersion in the high frequency range (108 Hz and 107 Hz resp.) was found which is related to the dipol reversal. The dispersion is described by a Gaussian distribution of relaxation times which show Vogel-Fulcher temperature dependences. The high value of the dielectric constant below the phase transition is related to the mobility of the domain walls. This mobility steeply decreases near the freezing temperature $T_f$ and therefore a sudden decrease in the susceptibility takes place. A nonlinear superposition of the piezoelectric resonance and the relaxational dispersion was observed.