Lipases are triacyl glycerol acyl hydrolases, which catalyze hydrolysis of esters, esterification and transesterification reactions, among others. Some of these enzymes have a large hydrophobic pocket covered by an alpha-helical mobile surface loop (the lid). Protein–protein interactions can occur through adsorption of two open lids of individual lipases. We investigated the conformation and oligomeric state of Thermomyces lanuginosus lipase (TLL) in solution by spectroscopic and mass spectrometry techniques. Information about oligomerization of this important industrial enzyme is only available for TLL crystals; therefore, we have done a throughout investigation of the conformation of this lipase in solution. SDS-PAGE andmass spectrometry analysis of size-exclusion chromatography eluted fractions indicated the presence of both monomeric and dimeric populations of TLL. The stability of the enzyme upon thermal and guanidine hydrochloride treatmentwas examined by circular dichroismand fluorescenceemission spectroscopy.Small angle x-ray scattering and ion mobility mass spectrometry analysis revealed that TLL is found as a mixture of monomers and dimers at the assayed concentrations. Although previous x-ray diffraction data showed TLL as a dimer in the crystal (PDB: 1DT3), to our knowledge our report is the first evidencing that TLL co-exists as stable dimeric and monomeric forms in solution.