An apparatus for producing and dispensing a reaction mixture starting from at least a first and a second reaction component that are under pressure in a first and a second container respectively, which apparatus comprises: - a mixing head for mutually mixing said first and said second reaction component; which mixing head is provided with an outlet for the mixture of both reaction components; - a first supply line for feeding the first reaction component from said first container under the pressure prevailing therein to the mixing head; - a second supply line for feeding the second reaction component from said second container under the pressure prevailing therein to the mixing head; - a first shut-off valve in said first supply line and a second shut-off valve in said second supply line, the first and the second shut-off valve each having an open and a closed position; - a third shut-off valve for shutting off the first supply line in said mixing head and a fourth shut-off valve for shutting off the second supply line in said mixing head, the third and the fourth shut-off valve comprising a common plunger which is configured to slide, in particular in its longitudinal direction, for opening and closing the third and the fourth shut-off valve simultaneously; - a first expansion chamber which is connected to the first supply line between said first and said third shut-off valve; - a second expansion chamber which is connected to the second supply line between said second and said fourth shut-off valve; - a pneumatic system (31, 33, 44, 45, 46 or 31, 44, 45) for the pressurisation and pressure-relief of the first expansion chamber and of said first supply line as from said first to said third shut-off valve, and for the pressurisation and pressure-relief of the second expansion chamber and of said second supply line as from said second to said fourth shut-off valve; and - a control system for operating said pneumatic system and said first, second, third and fourth shut-off valves for starting and stopping the production and dispensing of said reaction mixture, characterised in that said first and said second expansion chamber, respectively are formed by a first and a second pressure vessel having each two sides separated from each other by a membrane, of which a first side forms the first respectively the second expansion chamber, arranged to be filled with said first and respectively said second reaction component, and of which a second side (29, 30) is connected to said pneumatic system (31, 33, 44, 45, 46 or 31, 44, 45) to be filled with a gas, which gas is in contact through said membrane with the reaction component in the first side (25, 26) of the pressure vessel (13, 17), said first and said second pressure vessel being detachably connected to said pneumatic system (31, 33, 44, 45, 46 of 31, 44, 45) and to the first and the second supply line respectively. - a mixing head for mutually mixing said first and said second reaction component; which mixing head is provided with an outlet for the mixture of both reaction components; - a first supply line for feeding the first reaction component from said first container under the pressure prevailing therein to the mixing head; - a second supply line for feeding the second reaction component from said second container under the pressure prevailing therein to the mixing head; - a first shut-off valve in said first supply line and a second shut-off valve in said second supply line, the first and the second shut-off valve each having an open and a closed position; - a third shut-off valve for shutting off the first supply line in said mixing head and a fourth shut-off valve for shutting off the second supply line in said mixing head, the third and the fourth shut-off valve comprising a common plunger which is configured to slide, in particular in its longitudinal direction, for opening and closing the third and the fourth shut-off valve simultaneously; - a first expansion chamber which is connected to the first supply line between said first and said third shut-off valve; - a second expansion chamber which is connected to the second supply line between said second and said fourth shut-off valve; - a pneumatic system (31, 33, 44, 45, 46 or 31, 44, 45) for the pressurisation and pressure-relief of the first expansion chamber and of said first supply line as from said first to said third shut-off valve, and for the pressurisation and pressure-relief of the second expansion chamber and of said second supply line as from said second to said fourth shut-off valve; and - a control system for operating said pneumatic system and said first, second, third and fourth shut-off valves for starting and stopping the production and dispensing of said reaction mixture,