Thermal printing plates based on novolak, a phenol-formaldehyde condensation polymer, are now used by some 80% of the printing industry worldwide. Until recently, the image discrimination of these plates; i.e., the difference in the dissolution rate between exposed and unexposed areas was only moderate. About three years ago it was discovered that treatment of the plates with water vapor substantially improves image discrimination. This treatment is now an important manufacturing step, but there is no generally accepted explanation of the water vapor phenomenon. We venture to offer such an explanation and are exploring its consequences for the performance of the thermal plates.