A STRATIFIED fluid layer in which two components contribute to the vertical density distribution need not be stable even though the net density decreases upwards. If one of the components is unstably distributed, then molecular diffusion can release its potential energy—a phenomenon known as double-diffusion convection1. Considerable progress has been made over the past 10 years in the study of double-diffusive convection in situations where the properties vary in one direction (vertically) only. In the ocean, horizontal advection is nearly always important and its neglect in the previous laboratory experiments makes it difficult to apply the results in a wider context. A first step in the study of two-dimensional effects in double-diffusive convection was made by Turner and Chen2, and here I follow up one of their suggestions in detail in the hope that it may help to understand the layering which occurs when an outflow intrudes into an ambient fluid with the same density but different component concentrations3.