A high-temperature aquifer thermal energy storage (HT-ATES) experiment was conducted at the TestUM field test site in Northern Germany. For this purpose, an injection and extraction well were constructed in a Quaternary aquifer and used for heat injection and extraction. At both wells and at 59 measurement locations, temperatures were continuously recorded using 10 monitoring depths from 0 to 16.5 m below ground surface. The HT-ATES operation was experimentally simulated using an injection temperature of 81 °C as well as injection and extraction cycles ranging from single days up to two weeks and stagnant periods of corresponding length. Heat injection rates reached >70 kW, while a total of 155.6 MWh was injected during the total duration of the experiment. A heat balance was calculated at the injection and extraction wells as well as by interpolating the vertically resolved temperature measurements in the storage aquifer and the confining units. Results show that induced thermal convection significantly contributes to the heat loss of the HT-ATES. Return temperatures decreased from 81 °C at the start of an extraction phase to values as low as 20 to 40 °C, depending on cycle length. It was found that thermal recovery decreases with increasing heat residence time in the storage aquifer, with values ranging from 20 % to 50 %. A significant fraction of the injected heat moved into the upper confining unit by heat conduction and could not be recovered during the extraction cycles. For the short term one-day cycles, cycle stability could be demonstrated after about three cycles, while for the bi-weekly cycles the three experimental cycles only approximately reached cycle stability. Heat loss by groundwater flow is found to be small, due to the low groundwater flow velocity of about 0.09 m/d, and on the order of <5 % of heat injection and extraction rates. Furthermore, this work demonstrates that a detailed identification and quantification of the governing heat transport processes and heat losses in a HT-ATES system is possible when using a dedicated monitoring set-up as implemented at the TestUM field site.