The invention concerns an apparatus (1) for the dispersion of an expansion gas even in supercritical conditions, e.g. carbon dioxide, in a reactive resin, of the kind in which a reaction chamber having an input (27) for gas and an input (37) for resin is provided. Advantageously, the chamber is a dispersion and containment chamber made into a casing (2) of predetermined high resistance susceptible to sustain high pressure and is divided into two sections (6,7) by a head (14) of a dispersion and mixing cylinder-piston group (4) in fluid communication between themselves by means of at least one pouring passage (31, 36, 32, 39) provided with a static mixer (38), motor means (3) being provided for piston (34) control of said mixing cylinder-piston group (4). The invention also concerns a process for the formation of a polyurethane foam starting with the dispersion of carbon dioxide, even supercritical, in a reactive resin in which at least one initial dispersion and mixing controlled phase of the two components is provided in a dispersion and containment chamber under pressure divided into two sections (6,7) by a head (14) of a cylinder-piston mixing group (4) in fluid communication between themselves by means of at least one pouring passage (31, 36, 32, 39) provided with a static mixer (38) and in which adduction, dispersion and mixing occurs under high pressure (at least greater than 75 bar).