The compound Na2ZrSi4O11 has been prepared both by solid state synthesis and by the sol-gel technique via alkoxides. The structure of Na2ZrSi4O11 was investigated by the Rietveld method using X-ray diffractometer data. The powder diffraction data show evidence for a triclinic distortion of the basic monoclinic structure. The ionic conductivity has been determined in the temperature interval 130-560 degrees C using a.c. impedance measurements. The Na+ ion mobility is limited, with E(a) = 1.04 eV and sigma(300 degrees C) = 2-9 . 10(-7) S/cm. Samples with Ta partially substituted for Zr, according to the formula Na-2-x(Zr1-xTax)Si4O11 with x less than or equal to 0.2, showed the same evidence for a triclinic distortion. They had lower E(a) values (approximate to 0.75 eV) but only slightly higher conductivities at 300 degrees C.