The invention relates to a process to foam a water-base adhesive wherein a gas is dissolved or generated under pressure in the water phase essentially without the formation of foam. To apply the adhesive in the form of foam, the pressurized adhesive is released through a valve onto a substrate at atmospheric pressure, whereby the dissolved gas separates as gas bubbles. The invention relates further to an apparatus for the implementation of said process comprising a pressure-tight container for the adhesive wherein an overpressure is provided through a first valve, and at the outlet of said container a drain valve is placed. The apparatus further comprises a pressure-tight acid vessel for the acid which is furnished with a second valve at its inlet The outlet of said vessel is connected through a third valve to the container.