An improved method for testing the thermal performance of encapsulated heat storage materials in large nonstirred samples is presented. The method consists of measuring and comparing the energy loads of two identical thermostatted baths. Water, silicone oil or polyethylene glycol may be used as the working liquids in the baths, depending on the operating temperature. One of the baths operates with the sample of the encapsulated heat storage materials and the second operates with the working liquid alone under similar experimental conditions. Temperatures of the working liquids and the sample can be measured, and compared in the range 0-200-degrees-C. Differential measurements of this type provide valuable information on the thermal behaviour of the sample without the need to take into account the heat loss effects which are mainly difficult to determine. The test measurements for paraffin and high density polyethylene are discussed.