A new compound NH4H2(PO4)(0.52)(AsO4)(0.48) (ADAP) of the NH4H2PO4-NH4H2AsO4 system has been prepared by slow evaporation of an aqueous solution at room temperature. This mixed compound is isostructural of the tetragonal phases NH4H2PO4 (ADP) and NH4H2AsO4 (ADA) with the following parameters: a = 7.5920(13) Angstrom, c = 7.6260(2) Angstrom (Z = 4). Two kinds of disorder can be expected in this structure: one, statistical or dynamical, in the O-H-O hydrogen bond and the other related to the reorientational motion of NH4+ ions. Two-phase transitions at T-1 = 193 and T-2 = 418 K were detected and studied by differential scanning calorimetry, dielectric measurements and impedance spectroscopy. The evolution of the dielectric constant as a function of frequency and temperature revealed the first transition to be of first order and antiferro-paraelectric. It is assumed that the reorientation of NH4+ ions is more responsible for this phase transition than the hydrogen dynamics of the H2P(As)O-4(-) groups. The high-temperature phase transition at 418 K leads to a superionic-protonic phase. A jump in the conductivity is associated with this transition at an unusually high value of conductivity (4 X 10(-3) Omega (-1) cm(-1)) and low activation energy (E-a congruent to 0.3 eV) above 418 K.