Results from measurement of sap flow by heat balance sensors on the stem of a young oak tree (Quercus robur L.) revealed that thermal disequilibrium (i.e. heat storage) within the heated stem segment can introduce considerable errors in the measured sap-flow rates. The magnitude, sign and significance of these errors depend on the sap-flow rate and on the relationship between stem temperature and air temperature. Sap-flow rates were found to be more prone to errors caused by heat storage effects under low flow conditions than at higher rates of sap flow. Furthermore, daytime fluctuations of air temperature and stem temperature inside the heat balance sensor were either in phase when a low, or in opposite phase when a high sap-flow rate was passing through the stem of the young tree. To investigate this relationship, we developed an experimental set-up with cut stem segments through which tap water could be pressed. This set-up allowed the effects of air temperature and sap-flow rates on stem temperatures within heat balance sensors to be clearly separated. Good mathematical relationships were obtained and were successfully used to assess the relative importance of air temperature and sap-flow rate with respect to the fluctuations in stem temperature of the young oak tree. Based on the established relationships, a novel approach was put forward to correct for errors introduced into sap-flow calculations caused by heat storage effects if no measured data on stem temperature are available.