The electrical conductivity, crystal structure and phase stability of La0.99Ca0.01Nb1-xTaxO4-delta (x=0,0.1,0.2, 0.3, 0.4 and 0.5, delta = 0.005), a potential candidate for proton conductor for solid oxide fuel cells (SOFCs), have been investigated using AC impedance technique and in situ X-ray powder diffraction. Partially substituting Nb with Ta elevates the phase transition temperature (from a monoclinic to a tetragonal structure) from similar to 520 degrees C for x = 0 to above 800 degrees C for x = 0.4. AC conductivity of the La0.99Ca0.01Nb1-xTaxO4-delta both in dry and wet air decreased slightly with increasing Ta content above 750 degrees C, while below 500 degrees C, it decreased by nearly one order of magnitude for x = 0.4. It was also determined that the activation energy for the total conductivity increases with increasing Ta content from 0.50 eV (x = 0) to 0.58 eV (x = 0.3) for the tetragonal phase, while it decreases with increasing Ta content from 1.18 eV (x = 0) to 1.08 eV (x = 0.4) for the monoclinic phase. By removing the detrimental structural phase transition from the intermediate-temperature range, consequently avoiding the severe thermal expansion problem up to 800 degrees C, partial substitution of Nb with Ta brings this class of material closer to its application in electrode-supported thin-film intermediate-temperature SOFCs. Published by Elsevier B.V.