The in situ thermal response test provides an effective method to determine the ground thermal properties required for the design of a geothermal energy installation. This paper presents a unique compact testing device for in situ thermal response testing based on a previous, trolley-based unit. Modernisations in the presented new apparatus enable the time and cost of the test to be reduced and offer enhanced flexibility and application potential for in situ thermal response testing. The methodology adopted for the test, including the theoretical approach for the evaluation of test data, together with an outline of the design and workings of the apparatus are presented. A case study is also shown for the verification of the quality of the results. (c) 2007 Elsevier B.V. All rights reserved.