A method and apparatus for controlling the gas content of dispensed hot melt thermoplastic adhesive foam. Air or another relatively inert gas is mixed with molten thermoplastic adhesive, and the mixture is pressurized to form a gas and molten adhesive "solution". When the molten adhesive and gas "solution" is dispensed at atmospheric pressure, the gas is released from "solution" and becomes entrapped in the adhesive in order to form a homogeneous closed cellular adhesive foam. In order to control the gas content of the dispensed adhesive and gas "solution", in accordance with one preferred embodiment, the pressure drop across a restriction in the flow path of the solution is sensed, and a control circuit controls the addition of gas to the adhesive to maintain the sensed pressure drop at a desired level. Other sensing techniques can be used to measure a parameter of the dissolved gas/molten adhesive single phase liquid solution which is correlated to the dissolved gas content of the solution and substantially independent of the molten adhesive content thereof, such as measuring the transmittance of infrared radiation through the gas/adhesive single phase solution at a wavelength relatively heavily absorbed by the gas and unabsorbed by the adhesive.