We examined the gasification of cellulose, xylan, and lignin mixtures in supercritical water at 623 K and 25 MPa. Our results indicate that the lignin content significantly affects the amount and composition of the product gas. Thus, we surmised that cellulose or xylan is likely to function as a hydrogen donor to lignin. A set of equations developed to estimate the amount and composition of the product gas accurately predicted the actual results using only the lignin fraction as a parameter. This confirmed the importance of the lignin fraction effect on supercritical water gasification characteristics.