The heat balance method was evaluated in detail for its use in older mature grapevines with stems of 35 – 45 mm in diameter. Dye colouring of the xylem vessels revealed that even 21 year old grapevines did not show any development of heartwood and that xylem vessels of that age still have the capacity to transport water. A comparison of weight loss of potted vines on a balance and sap flow measurements demonstrated that the heat balance system reflected rapid changes in flow rate without any time delay. However since even 20 year old xylem vessels of grapevines have the capacity to conduct water the heater band was not able to heat the sap in all year rings evenly. Apparently at low flow rates this effect was small and sap flow was calculated correctly. With increasing flow rates large thermal heterogeneities developed upsetting the calculation of the heat balance and mass flow. Consequently actual sap flow was overestimated by 50 to 100% at high flow rates. This could be attributed to thermal gradients in these relatively thick stems excluding the use of this technique for measurements of long term as well as short term water use patterns in older grapevines.