This work reports the study of the dielectric relaxation of betaine arsenate (BA) and of strongly deuterated betaine arsenate (DBA92) below room temperature for different values of d.c. electric fields, in the frequency range of 50 Hz-13 MHz. The main contributions for dielectric constant come from low-frequency relaxation modes. Some evidence for the existence of a new antiferroelectric phase in BA was found, stable in a short range of temperatures.