The application of Microsoft Excel Solver routines to analyze data from a thermal-probe placed in a number of samples is described. The successful application of the Solver routine to the thermal-probe temperature time data requires the selection of an appropriate time-interval. The various critical times are discussed and a method proposed for selecting the appropriate Solver routine and the time-interval over which the experimental data must be analyzed. Values of the sample thermal conductivity, the thermal diffusivity and the probe to sample conductance are reported for a stainless-steel probe in unused mineral oil, and paraffin wax and for a copper probe in Magna904 and PTFE. The results for a commercial probe in agar agree with accepted data for water and the results for a clay–straw mixture demonstrates the method applied to a good insulator.