Disruption mitigation is mandatory for ITER in order to reduce forces and to mitigate heat loads during the Thermal Quench (TQ) and from runaway electrons. A fast Disruption Mitigation Valve (DMV) has been installed at JET to study mitigation by Massive Gas Injection (MGI). Different gas species and amounts have been investigated with respect to timescales and mitigation efficiency. Halo currents as well as sideways forces during VDE are successfully reduced by MGI. About 50% of the thermal energy is dissipated by radiation during a MGI disruption. The radiation is homogeneously distributed poloidally, with a peaking factor below 1.5 during thermal and current quench. Significant poloidal and toroidal peaking of up to 2.5 is observed before the thermal quench. Runaway generation is successfully avoided by the injection of mixtures of Ar or Ne with D2. In contrast, injection of pure Ar leads to runaway generation even at low toroidal magnetic fields. Although, runaways can be safely avoided by MGI in JET disruptions, the density reached is a factor 50 below the critical density for avalanche suppression, which will be essential in ITER.