Multivariate analysis was applied to the volatile content data obtained by Yoshida and his co-workers (e.g., Yoshida et al., 1971) to understand magma genesis in subduction zones. A statistical analysis was conducted by the methods of principal component analysis (PCA), factor analysis and hierarchical aggregative cluster analysis. The data can be interpreted on the basis of three principal components and three common factors. The first factor implies the contribution of seawater to magma genesis, which may take place through volatile release from the downgoing plate to the overlying upper mantle wedge. The second factor corresponds to the contribution of the mantle component with higher scores in volcanic rocks on the far side of the volcanic front. The third factor suggests the separation of FeS phase from silicate phase.