A total of 24 temperature profiles from ~92 to 55 km were obtained from falling sphere flights in Longyearbyen (Svalbard, 78¿N) from 16 July to 14 September 2001. The thermal structure of the upper mesosphere during the summer season (here from mid-July to 23 August) is characterized by very low temperatures and little variability. The mesopause temperature decreases slightly from ~130 K in mid-July to 126--128 K in late July/beginning of August. The mesopause altitude in summer is ~89 km. Compared to 10¿ further south (69¿N, And¿ya), the mesopause temperature is very similar in mid-July but is significantly colder by 6--8 K in the second half of July and in August. Part of this difference (especially in late August) is due to the later transition from summer to winter in Longyearbyen. The mesopause altitude is higher by approximately 1 km at Longyearbyen compared to And¿ya. At 82 km, the temperature in summer is very close to 150 K, very similar to other Arctic and Antarctic stations (equithermal submesopause). The temperatures in the upper mesosphere are significantly lower compared to COSPAR International Reference Atmosphere (CIRA, 1986) by up to 20 K. Assuming model water vapor concentrations, we derived the degree of saturation of water vapor (S). In summer, there is an extended altitude range (82--92 km) with supersaturation (S > 1). Occasionally, very high supersaturation was derived (S > 100). Our temperature measurements are in general agreement with the occurrence morphology of polar mesosphere summer echoes (PMSE). However, double layered structures frequently observed in PMSEs are not a prominent feature of the temperatures in the upper mesosphere.