Phase equilibrium pressure temperature (pT) conditions in methane clathrate hydrate (CH4 hydrate) systems with miscible additives (monosaccharide and sugar alcohol) were measured by an isochoric method using xylose (monosaccharide) and xylitol (sugar alcohol) as additive molecules. The equilibrium pT data were collected in the pressure range of 3-7 MPa and an additive mole fraction range in aqueous solution, x(additive), of 0.006-0.03. The equilibrium pT conditions in both systems existed in higher-pressure regions than that for a additive free CH4-water system. Crystals formed in both the systems were identified as structure I CH4 hydrates by powder X-ray diffraction and Raman spectroscopy measurements. Considering the phase boundary and crystallography results, xylose and xylitol acted as thermodynamic inhibitors for clathrate hydrate systems.