Experimental simulations were carried out to determine hydrodynamic flow structures and phase mingling mechanisms of a completely submerged shallow water volcanic vent with the objective of understanding how flow conditions ultimately affect the character and explosiveness of hydromagmatic eruptions. Negatively buoyant jets were produced by injecting glycerin-water mixtures (magma surrogate) into silicon oil (water surrogate) and imaging the flow structure using a laser/particle image velocimetry system. The experiments were designed to evaluate only the initial mingling processes of magma and water and do not account for subsequent vaporization of water and heat transfer effects. The onset of phase mingling is strongly controlled by the Richardson number of the jet, which governs the flow structure and stability. At Ri <0.90 an oscillatory collapsing fountain is developed, which entrains ambient fluid by vortex rollup and collapsing margins. On the basis of the experimental results the incorporation of water into basaltic magma discharged from a submarine vent of the order of 10 m in diameter is predicted to occur when the volume discharge rate is in excess of 460 m3/s.