Abstract: A process for the simultaneous, in-situ determination of thermophysical properties, primarily for determination of the thermal conductivity and thermal diffusivity, according to which in a certain volume of the material to be tested a temperature field varying with time is brought about by perturbation due to heating and this temperature field is measured, then certain properties are determined by calculation from the obtained temperature data and the known heating power input spherical isotherms are formed in the material to be tested in a way, that temperature change of at least one degree Celsius is brought about at the measuring points which are arranged along a straight line passing through the heat source and intersecting the isotherms along their diameter, and a measuring probe formed as a rod-type body made of poor heat conductive material having a heating element and measuring points, wherein the heating element is formed as a point-like heat source.