'ABSTRACT Posidonia oceanica and Vallisneria americana are lignocellulosic biomass found in sea and fresh water, composed of a sufficient amount of cellulose 49% and 45% respectively, which can be converted to monosaccharides and fermented to produce ethanol. In this study the effect of alkaline pretreatment, dilute acid pretreatment and sonication followed by enzymatic hydrolysis for the conversion of polysaccharides present in the cellulose fraction of Posidonia oceanica and Vallisneria americana to monosaccharides were evaluated. The pretreatments were compared based on the amount of Total carbohydrate and the energy consumed for the pretreatment, so also the optimum pretreatment conditions were selected based on the maximum amount of Total carbohydrate extracted and best in terms of energy consumption. Pretreatment with 1% H2SO4 (sulfuric acid) concentration, at 90oC reaction temperature and 60 minutes reaction time followed by sonication, gave the highest yield of Total carbohydrate for both Posidonia oceanica and Vallisneria americana 172.9mg/g and 127.7mg/g respectively using 5g/l feedstock concentration. The residue obtained after pretreatment was subjected to enzymatic hydrolysis using cellulase with enzyme activity of 15 FPU/g (filter paper unit) of solid, in a citric buffer 0.01M (pH 4.7) at 50oC, 150rpm with 5% w/v solid loading of biomass. The highest yield of total reducing sugar were 424.07mg/g and 418.65mg/g obtained from Posidonia oceanica and Vallisneria americana respectively. Fourier transform infrared spectroscopy and X-ray diffraction analysis were carried out to investigate the effect of dilute acid pretreatment, sonication and enzyme hydrolysis in the solid biomass. Key words: pretreatment, total reducing sugar, water grasses'