Spectroscopic parameters in an ensemble of the weakly bound dimers are derived making use of the quantum and classical statistical average over thermally excited rovibrational states. Argon dimers and oxygen dimers are taken as examples. It is shown that thermally averaged characteristics of dimers rapidly change as temperature of a gas rises. In an ensemble of quasi-diatomic tightly bound dimers, the high-temperature limit of the effective dissociation energy amounts to roughly D 0 /3, where D 0 is the dissociation energy from the ground state. It is demonstrated that the effective rotational B eff (T) constant for the oxygen dimers statistically averaged at near room temperature is merely one half of the ground state B 0 value. This conclusion is in agreement with recent spectroscopic observations. Rotationally resolved spectroscopic probe of (O 2 ) 2 in the slit-nozzle expansion resulted in B 0 ฯญ 0.095 cm ฯช1 , while bandshape analysis of the room-temperature collision-induced absorption in the oxygen fundamental allowed an estimate of B eff ฯท 0.038 cm ฯช1 . The role of anisotropy of intermolecular interaction and the formation of various types of metastable dimeric states are discussed.