The stability of a double diffusive system comprising of continuous gradients in temperature and salinity has been examined with respect to finite amplitude disturbances. On the basis of existing experimental data it is shown that for the particular case where salinity provides the stabilizing gradient (the ''diffusive'' mode) such instability will occur when the system is still stable to infinitesimal disturbances. A numerical model that incorporates the effects of the double diffusive instability together with other mixing processes that occur in natural and man-made water bodies has been developed. Data from two salt gradient solar ponds, where the finite disturbance is provided by the differential heating that occurs at the shallow sidewalls are shown to agree well with the model results. While the exact details of the instability may eventually be shown to differ from those assumed in the model, it appears beyond question that in the presence of finite amplitude disturbances, the accepted infinitesimal stability criterion is of no real significance. ¿American Geophysical Union 1987