Injection devices are provided for injecting additives into a refrigeration or air conditioning systems, which include a tubular element defining an inner chamber, the inner chamber having a volume at least equal to the volume of the quantity of additive to be injected. An inlet fitting is attached to a first end of the tubular element; an outlet fitting is attached to a second end of the tubular element. The devices are also provided with an additive inlet port, distinct from the inlet and outlet ducts, fluidically communicating with the inner chamber and configured for removable connection to an additive injection device. The inlet port is engaged by a one-way valve suitable for allowing the passage of the additive only from the injection device to the inner chamber. Kits and methods are also provided.