Heat shock cognate protein 70 (HSC70) is a 71 kDa molecular chaperone belonging to the ubiquitous family of heat shock proteins 70 (Hsp70). The representatives of this protein family are molecular machines with ATPase activity facilitating correct folding of spatial protein structure, both in normal and stressful conditions (e.g., hypoxia, heat shock, pH fluctuations etc.) In addition, HSC70 fulfills several specific functions not shared by other Hsp70 homologs, like uncoating of clathrin-coated vesicles, protein transport into organelles and targeting pro-teins for lysosomal degradatifon. For many decades it was known that Hsp70 family members can form dimers and possibly higher oligomers. The dimerization of Hsp70 and its functional meaning remains elusive to this day, even though multiple attempts were made to tackle its struc-tural and functional aspects. In addition, the positive impact of J-domain proteins (JDPs, or Hsp40) on Hsp70 dimerization was observed in earlier studies. In this work, chemical cross-linking, high resolution Fourier transform mass spectrometry (FTMS), heavy nitrogen (15N) isotop-ic labeling and sophisticated data analysis were utilized to structurally investigate the HSC70 dimer population. The collected distance restraints imposed by the crosslinker length allowed to build structural models of HSC70 monomers and dimers by AlphaLink 2. Based on the data ob-tained, we were able to distinguish ADP-state and ATP-state dimer conformations for HSC70, which co-exist in the equilibrium. Moreover, we observed the acceleration of HSC70 ADP-state formation in the presence of DNAJB1 co-chaperone, with enhanced ATP hydrolysis at higher DNAJB1 concentration. Our findings indicate that the HSC70 dimer population is heterogeneous in vitro and different types of dimers are formed depending on the incubation conditions and the presence of co-chaperone.