The defect behaviour of the low melting salt potassium thiocyanate has been studied. Ionic conductivity measurements were made on 'pure' single crystals and compressed pellets of KSCN, Diffusion studies were carried out on single crystals using radioactive tracers. The crystals were grown from the melt using zone refined KSCN which was shown by mass spectrographic analysis to have a low impurity content. The conductivity of the KSCN was measured by an a.c. method. The conductivity and activation energy were reproducible for several crystals giving two regions defined by the following equations:- for the temperature range 130C - 165C and for the temperature range 50C - 120C. The usual impurity dependence was not observed for the conductivity of KSCN pellets containing various amounts of lead impurity. The conductivity increased throughout the whole temperature range. The effects are discussed in terms of impurity dissolution and the relative number of defects. The diffusion of 42K was measured by a sectioning method and is represented by the equation:- for the temperature range 40 - 160C. The formation of free defects appears to be insignificant in controlling the diffusion. Comparison of the diffusion and conductivity results by the Nernst-Einstein relation shows that there is a large contribution to the diffusion by a neutral species, Vacancy pair diffusion is considered to be the cause of this disparity and anion diffusion work confirms this theory, the S14 CN- diffusion being of the same order as the 42K+ diffusion. Low values obtained for the impurity diffusion of 210Pb+ and 35S2- ruled out any significant contribution by impurity vacancy complexes. The phase transition was studied in detail by differential thermal calorimetry and by detailed conductivity and capacitance measurements. The temperature of the transition was not affected by added impurities.