These datasets provide data for the lowest part of a new composite profile GOS18 from Lake Gościąż in central Poland. The composite profile was established using sediment cores recovered in 2015 and 2018 with an UWITEC Piston Corer at 19.6-21.5 m water depth (Bonk et al., in press). Our data covers the time interval from the onset of lacustrine sedimentation in the late Allerød to the early Preboreal. Since Lake Gościąż comprises a continuous, seasonally resolved (varved) and exceptionally well-preserved archive of the Younger Dryas (YD) climate variation, it is highly suitable for detailed investigations of lake system responses during periods of rapid climate cooling (YD onset) and warming (YD termination), respectively. Chironomidae head capsules (hc) were utilized to reconstruct the mean July air temperature from the late Allerød to the early Preboreal in Lake Gościąż (central Poland). Sample resolution ranges from 0.5 to 6 cm. Two different training sets were used for the reconstruction - the Swiss-Norwegian-Polish Training Set (SNP TS) (Kotrys et al. 2020) and the East European TS (EE TS) (Luoto et al. 2019). Both use the Weighted Averaging-Partial Least Squares transfer function (WA-PLS).The dataset incorporates the composite depth and age [BP] for the sample midpoint, as well as for both training sets, respectively, the chironomid-inferred mean July air temperature, the standard error and the squared chi-square distance between the fossil sample and its closest modern analogue in the respective training set.