NaxSi136 clathrate-II single crystals with x = 2.9, 5.1, 8.2, and 14.7 were prepared by a two-step process. In the first step, Na24Si136 single crystals were grown from the precursor Na4Si4 by reaction of the vapor phase with spatially separated graphite in a closed volume. In the second step, the Na24Si136 single crystals were subjected to thermal decomposition in a nitrogen atmosphere at 10 Torr and 405 degrees C. The Na content was controlled by the duration of thermal decomposition. The structural properties were investigated using single-crystal X-ray diffraction and compared with those of single-crystal Na24Si136. The quality of the obtained products also allowed for low-temperature transport property measurements on agglomerates of crystals allowing for an investigation into the low-temperature electrical and thermal properties as a function of Na content.