The oxidative conversion of furfural to C4 carboxylic acids was investigated using both model and biomass-derived systems. Furfural was generated from sweet sorghum hydrolysate via biphasic acid-catalyzed dehydration, and its recovery into the organic phase was found to be efficient. However, organic acids, such as acetic acid and formic acid, present in the biomass-derived solution significantly inhibited downstream oxidation. Comparative experiments using model compound systems revealed that these acids reduced succinic acid yields by up to 16 mol%, likely due to the formation of peracetic acid and performic acid or competition for the oxidant. Further parametric analysis identified optimal reaction conditions of 70 °C and acid catalyst and oxidant concentrations of 1:1 and 4:1 the molar ratio of furfural, respectively. Under these conditions, succinic acid formation was favored by 2(3H)-furanone, whereas maleic acid formation was associated with elevated temperatures and 2(5H)-furanone intermediates. These results highlight the need to control biomass-derived inhibitors and optimize reaction conditions to improve the yield and carboxylic acid selectivity in furfural-based oxidation systems.