The atmospheric oxidation of monoterpenes leads to the formation of secondary organic aerosol (SOA). While numerous works have been carried out in the past to characterise SOA at a molecular level, the structural elucidation of SOA compounds remains challenging owing to the lack of authentic standard compounds. In this work, the structures of α-pinene originating dimeric esters in SOA with m/z 357 (C17H25O8-) and m/z 367 (C19H27O7-) were characterised using UPLC/ESI(-)IMS-TOFMS2 (ultra-performance liquid chromatography coupled to ion mobility spectrometry tandem time-of-flight mass spectrometry). The measured collision cross-section (ΩN2) values were compared to theoretically calculated ΩN2 values. Selected product ions of dimeric compounds and the authentic standard compounds of product ions were subjected to CO2-IMS-TOFMS for more detailed structural characterisation. Our results were consistent with previously reported subunits of the m/z 357 (terpenylic acid and cis-pinic acid), and the m/z 367 (10-hydroxy-cis-pinonic acid and cis-pinic acid) ions. The measured and calculated ΩN2 values of m/z 367 ions further support the conclusion of earlier structural characterisation; however, the structure of the m/z 357 ion remains vague and requires further characterisation studies with a synthesised reference compound.