The traditional way of making emulsions is via the input of extra mechanical energy, but there is another mechanism of emulsification that is entirely thermodynamically controlled. Experimental results are presented elucidating the consequences of this spontaneous emulsification for heterogeneous reaction systems. Special emphasis is placed on aqueous heterophase polymerization. We present the results of unusual experiments in oil–water systems that fundamentally changed our view of the mechanism of emulsion polymerization.