a method and apparatus for controlling dampening water utilized to form a thin film on the printing roll of the type , wherein the dampening water is circulated through the printing machine and a water tank , the level and temperature of the dampening water being maintained at definite values . furthermore the ph value of the dampening water is measured and an acid or alkali is added to the dampening water in accordance with the result of the measurement so as to control the ph value at a definite value .