Abstract: Effective control of warp in the manufacture of corrugated paperboard sheets on standard corrugator equipment is achieved by the on-line application of a regulated quantity of moisture to one or both sides of the completed corrugated paperboard web after it has been heat-bonded together, while it is still hot from the heat-bonding operation, and prior to its being cut into individual corrugated paperboard sheets. By proper selection of the quantity and placement of the moisture application treatment, it is possible to counterbalance the net effect of the warp-producing conditions developed within the bonded paperboard web prior to and during its bonding operation, so that the resulting paperboard sheets leaving the corrugator will be consistently substantially flat. The moisture application treatment is carried out with an adjustable moisture spray system designed to selectively apply to either or both sides of the bonded paperboard web a variable regulated quantity of moisture.