The complex dielectric constant of (CH3)(2)NH2H2AsO4 was measured in a frequency range between 10 MHz and 13 GHz. Dielectric dispersion was found to be very close to the monodispersive Debye e type. The critical slowing-down phenomenon of polarization was observed near the phase transition temperature of 274.5 It (T-c). Dielectric constants epsilon in crystals of (CH3)(2)N(H1-xDx)(2)(H1-xDx)(2)AsO4 system were measured at 100 kHz as functions of temperature. The ferroelectric phase transition temperature increases with increasing deuterium concentration x. The temperature dependence of the relaxation time tau in the paraelectric phase can Le well explained by the quasi-one-dimensional dynamic Ising model. The activation energy was estimated to be E-a = 64.3 meV.