The mass transport during pyrolysis of the degradation product of a polymeric binder used for ceramic molding is analyzed. Two series resistances to transport are incorporated : the diffusion of the product of degradation through its parent polymer during decomposition and the permeation of vapor through a static powder bed used to support the molding. The model predicts the critical rate of heating, Z(c), above which defects caused by boiling of the organic phase occur at the center of the molding. For the system modeled, the permeability of the powder bed has little effect on Z(c) if the moldings are small but this is not the case for large moldings where well-packed fine powders should be avoided and the thickness of the powder bed should be kept to a minimum.