We present here an investigation on the annual cycle of the carbonaceous fraction of the Po Valley (Italy) fine (d<1.5 μm) aerosol. Water-soluble organic compounds (WSOC) characterisation was accomplished using a simplified procedure, described in the paper, of a previously proposed methodology. The new simplified procedure allows the analysis of large sets of samples, which was difficult to achieve with the previous complex and labour-intensive method. The results of this study show that the aerosol total carbon (TC) concentration follows the same annual trend as the aerosol inorganic ion constituents, characterised by lower concentration values during the summer and higher concentrations in winter. The total aerosol organic carbon (OC) represents from 90 to 97% of TC, the rest being accounted for by elemental carbon. Within this study, we studied in particular the aerosol WSOC, a class of chemical compounds for which present knowledge is rather limited. In our samples, WSOC account for between 38% (in winter) and 50% (in summer) of OC, in terms of carbon. WSOC were fractionated into three main classes: (a) neutral/basic compounds; (b) mono- and di-carboxylic acids; (c) polyacidic compounds. The three fractions together accounted for an average 87% of total aerosol WSOC. The acidic compounds (mono- and di-carboxylic acids+polyacidic compounds) were found to be far more abundant than the neutral ones in all seasons, with polycarboxylic acids being the most abundant class of WSOC in spring, fall, and winter, while the mono/di-acids are dominant in summer. The main structural features of aerosol WSOC, revealed by proton nuclear magnetic resonance, suggest that WSOC are composed of highly oxidised species with residual aromatic nuclei and aliphatic chains. In particular, neutral compounds are mainly polyols or polyethers, mono/di-acids are mainly hydroxylated aliphatic acidic compounds, and polyacids are highly unsaturated compounds of predominantly aliphatic character, with a minor content of hydroxy groups.