Acidic hot water flowthrough pretreatment (AHWF) is an attractive approach primarily because of its high efficiency for sugar recovery. However, a significant portion of carbon content in the solubilized fractions from lignin depolymerization and monosugar degradation has been studied to a lesser degree. Herein, we investigated the solubilized non-carbohydrate products from a series of flowthrough pretreatments of poplar wood by water-only or very dilute acid (0.05% to 0.1% w/w, H2SO4) at different temperatures (220 to 280 °C) and flow rates (10 to 62.5 mL/min). Results revealed that tailoring reaction parameters (temperature) and operational parameters of reactor (flow rate) without adding expensive catalysts can selectively produce specific non-carbohydrate compounds. Up to 50.9% selectivity of vanillin and 45.0% selectivity of syringaldehyde were obtained at 240 °C for water-only treatment with flow rates of 25 mL/min and 62.5 mL/min, respectively. Lower temperature (e.g., 220 °C) was favorable for the formation of coniferyl alcohol, with the highest selectivity of 36.2%. Higher temperature (e.g., 280 °C) or lower flow rate (e.g., 10 mL/min) led to the formation of varied other aromatic compounds and HMF. Adding very dilute acid (0.05% to 0.1% w/w, H2SO4) into the water-only system considerably enhanced the formation of HMF with up to 66.7% selectivity.