The effect of partially substituting praseodymium for lanthanum in the superconducting materials La//2// minus //xSr//xCuO//4 has been investigated. Powder neutron diffraction work demonstrated that only minor structural changes occur; however, marked differences in electrical properties were found. Compounds of the general formula, La//1//. //6//2// minus //xPr//xSr//0//. //1//8CuO//4, synthesized using a precipitation method from Pr**3** plus compounds, were found to be semiconductors in the range 0 less than x less than 0. 27. In the La//1//. //6//2// minus //yPr//0//. //1//8Sr//yCuO//4-phase region a change from semiconducting to superconducting behavior was found on increasing the level of strontium doping.