In this work is presented online monitoring of gasification of durian (Durio zibethinus), onion (Allium cepa L.) and garlic (Allium sativum L.) as real biomasses containing sulfuric compounds, in sub and near critical water (250, 300 and 350 °C) by Lithium Ion Attachment Mass Spectrometry. Seven sulfuric compounds were detected in the resulting gas phase: H2S, ethanethiol, 1-propanethiol, methyl phenyl disulfide, propyl disulfide, diethyl trisulfide and dipropyl trisulfide gases. Allicin and alliin were successfully detected from onion and garlic samples. The effect of the subcritical conditions used on the release of sulfur heteroatomic compounds from real biomass was examined. Ca(OH)2 was adopted as an alkali reagent to suppress the emission of sulfuric compounds to the gas phase. Extraction was promoted and decomposition processes were observed for durian when Ca(OH)2 was added. For onion and garlic samples the decomposition process was prevailing with the addition of Ca(OH)2.