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This study presents the catalytic valorization of vegetable waste into levulinic acid (LA) over a solid Brønsted acid, Amberlyst 36, in aqueous solution with/without polar aprotic solvent (dimethyl sulfoxide, DMSO). With the aid of microwave heating, the cellulosic structure in the vegetable started hydrolysis at 120 °C. Increasing temperature to 135–150 °C accelerated the yield of LA to 13–17 C mol% in 5 min in aqueous solution due to the reduction of mass transfer limitations. At the same time, the generation of insoluble humins became more rapid and significant, leading to lesser amount of total soluble products. In comparison, in the DMSO-water mixture, the early-stage reaction produced up to 17 C mol% of hydroxymethylfurfural (HMF), which was gradually rehydrated to LA with increasing reaction time. This probably illustrated the effects of DMSO, i.e., increased accessibility of glucose to protons for rapid HMF …