Furanoids are a major compound class emitted during biomass burning and are highly reactive toward hydroxyl radicals (OH), suggesting their oxidation may contribute to secondary organic aerosol (SOA) formation. Previous furanoid studies focused on gas-phase kinetics, leaving SOA formation poorly understood. Here, we investigate the gas-phase oxidation and SOA formation from furfural, 2-methylfuran, and 3-methylfuran reactions with OH radicals in the presence of nitrogen oxides at RH&lt;5 %. Furfural exhibits distinct oxidation chemistry compared with methylfurans: furfural alkyl radical preferentially reacts with NO₂, whereas methylfuran oxidation proceeds via peroxy radical (RO₂)+NO channel to form alkoxy radicals. Methylfuran isomers also differ in oxidation product speciation, with 2-methylfuran generating C₄ and C₅ compounds, whereas 3-methylfuran predominantly forms C₅ compounds. C₄H₂O₃, potentially maleic anhydride, increases continuously during all furanoid oxidations, supporting its potential as an aged biomass burning marker. Furfural produces the highest SOA mass and yield (66.3 &micro;g&sdot;m⁻&sup3;, 9.8 %), followed by 2-methylfuran (10.3 &micro;g&sdot;m⁻&sup3;, 1.4 %) and 3-methylfuran (7.2 &micro;g&sdot;m⁻&sup3;, 1.1 %). Compared with methylfuran SOA, furfural SOA shows stronger signatures of lower-volatility oligomers, with higher contributions from large <em>m/z</em> fragments, a greater degree of unsaturation, broader thermograms, and more reduced CHON species (O/N&lt;3). Accordingly, estimated particulate organic nitrate fraction is highest for 2-methylfuran SOA (~45 %), followed by 3-methylfuran (~36 %) and furfural (~11 %). These results show that both substituent type and position should be considered when representing furanoid oxidation in atmospheric models.