The thermal diffusivity α of pure lithium-ammonium sulphate (LAS) crystals is measured along the three crystallographic directions in the temperature range 300–500 K using the plane temperature wave technique. The effect of doping LAS crystals with some divalent metal ions on the temperature dependence of α in both the ferroelectric and paraelectric phases is discussed. The average length of the distorted regions formed by the dopant in the lattice of LAS is calculated. The thermal conductivity of LAS crystals is calculated in the same temperature range. The phonon mean free path in both the ferroelectric and paraelectric phases is estimated and the results are discussed in the light of the difference between the ionic radii and the concentrations of the dopant ions in LAS.