KH2PO 4 (KDP) and NH4H2PO 4 (ADP) crystals are protonic conductors. Murphy [1] considered the presence of L and D defects in the H2PO4 sublattice to explain the conduction mechanism in ADP. However, Harris and Vella [2], in analogy with the Har r ing tonStavelley [3] model for NH4C1 , considered that the proton migration in ADP crystal occurs in the NH4 + sublattice (A defect) in addition to the H2PO 2 sublattice. Our earlier studies on the electrical conductivity and dielectric loss of MoO I -doped KDP crystals [4] were explained in terms of L defect mechanism. ADP being isomorphous to KDP, the above mechanism should also hold here. In the present study the electrical conductivity and dielectric loss studies of MoO ] -doped ADP crystals were undertaken to verify whether proton migration in the NH2 sublattice also contributes to the conduction in the ADP crystal, in addition to the migration in the H2PO4 sublattice. Because crystals of MoO] -doped ADP were grown by the slow evaporation of the saturated solution. K2MoO 4 was used as dopant. The presence of the impurity MOO4:in the crystal was detected by studying its optical absorption. A Cary-14 spectrophotometer was used to record the optical absorption. The crystal faces were polished and coated with silver paint to give good electrical contact between the crystal and the electrodes for the transport studies. The resistance of the sample was measured using a Keithley electrometer model 610C. The temperature of the crystal could be controlled to within 4-0.5 K. For dielectric loss measurements, a GR1615 Impedance Bridge was used. The connections between the electrode and the bridge were made through lowloss cables. The optical absorption of the doped crystals showed two bands at 230 and 210 nm. The band positions agree very well with the earlier reported spectrum of MoO42 in solution [5]. The above study confirmed the presence of MoO] in the ADP crystal. Fig. 1 shows the plot of log (aT) against l IT for undoped and MoO]-doped ADP crystals. The conductivity plot shows the following characteristics: (i) there is a knee around 94 ° in case of the pure ADP crystal in agreement with the reports by Murphy [1] and Perino and Wentreck [6]; (ii) the conductivity of