The "steady-state" non-linear optic polarization arising in lithium sulphate monohydrate crystals under the influence of the short pulse laser irradiation has been investigated in the temperature range of 95-305 K. The reversal of the sign of the components of the generalised non-linear susceptibility tensor % has been found at about 105 K. The change of the photoresponse kinetics and a drastic temperature dependence of the components Xijk in the temperature range of 95-105 K, as well as the change of the spectral dependence of the photoresponse were observed.