Abstract: The invention relates to a method and apparatus or plant for preparing a disinfestation or disinsection fluid which contains an inert fluid and phosphine obtained from the hydrolysis of a water-decomposable metal phosphide. According to the invention to the inert fluid H.sub.2 O is added, the inert fluid/H.sub.2 O mixture brought into contact with the metal phosphide for the reaction and the resulting disinfestation fluid separated from the solid reaction residue. The mixture of inert fluid and H.sub.2 O is brought to react either in liquid or in gaseous state with the metal phosphide. A simple method of preparing the disinfestation fluid resides in that in an opened pressure container (1) calculated amounts of metal phosphide (2), solidified inert fluid (3) and water ice (4) are layered over each other in that order, the pressure container (1) closed and a temperature equalization with the ambient temperature allowed to take place.