A process to apply a foamable water-based adhesive which is an aqueous polymer solution or dispersion, wherein a) a first aqueous polymer solution or dispersion which has a pH-value above 6 and which contains a dissolved or dispersed salt, which liberates a gas when it is combined at a pressure of 1.01 bar (1 atm) with an acidic solution with a pH-value of lower than 5 is provided in a first container, b) a second aqueous polymer solution or dispersion which comprises an acid and which has a pH-value of lower than 5 is provided in a second container, c) the first and the second aqueous polymer solution or dispersion are combined in a pressure-tight container having an outlet valve, d) the combined aqueous polymer solution or dispersion, which forms the water-based adhesive, is released from the pressure-tight container through the outlet valve onto a substrate at a pressure lower than the pressure within the first container, whereby the gas liberated from the dissolved or dispersed salt during and after step c) forms gas bubbles in the aqueous polymer solution or dispersion leaving the valve, thus foaming the aqueous polymer solution or dispersion. The two-component foamable water-based adhesive which can be used in this process forms also part of the invention.