This paper presents recent developments of the transient plane source techniques, and a discussion about their ability to monitor structural changes. The application demonstrated here is the monitoring of milk fat by measurement of the thermal conductivity. A discussion on various issues such as probe design, probe size and thermal contacting area on the influence on measurement sensitivity and accuracy, are covered, as well as the connection between the thermal depth of probing and measurement time. The present results indicate that it is possible to design a thermal conductivity probe, capable of recording the fat content in milk with a sensitivity of better than 0.1%, within a total measurement time of 1 s. In these measurements, the thermal depth of probing was 0.8 mm with a sensor radius of 2 mm, corresponding to an active sample volume of 40mm(3). If used as an online quality control indicator, these measurements can be repeated continually every 20s with a single probe. (c) 2005 Elsevier B.V. All rights reserved.