A salt gradient solar pond is a large thermo-haline double-diffusive system of depth up to 3 m, having a lower convecting or storage zone with about 20% salt by weight, and a non-convecting uniformgradient zone above providing insulation. Heating by solar radiation is partly within the liquid and partly at the base. The principal fluid mechanics problems are growth of the lower convecting zone, emergence and growth of an upper convecting zone and the possible breakdown of the gradient zone into a number of convecting zones simultaneously. It is shown that the well accepted model of double-diffusive convection for ‘strong’ heating from below a stable salinity gradient does not apply at the very modest level of solar radiation, more especially when part (or even all) of the heat is generated within the fluid. New models are developed in which buoyant thermals play causal roles (as, indeed, they do in the case of ‘strong’ heating). Thermals from all three kinds of boundaries are investigated, and it is found that although there is a limited quantity of fluid in a thermal, connection is retained with the source during its lifetime. Thermals from the solid base are of the well known axisymmetric kind. Those from the boundary with air, and from liquid of a different density, the so-called ‘free’ boundaries, are shown to be two-dimensional and Gaussian in profile. Thermal ranges are found to be well defined and increase according to environmental (water) temperature, with a common form of range equation. The equation is calibrated experimentally for each type of thermal. For a given environmental temperature, axisymmetric thermals have by far the longest range whilst Gaussian thermals from a boundary with air have the shortest. If thermals do not reach the opposite boundary there is local heating or cooling ; if they do, there is boundary erosion and zone growth, that is, penetrative convection. Each convecting zone has two sets of thermals, upwards and downwards, which are the principal means of heat transfer. This is illustrated for the particular case of a solar pond. Finally, it is shown that the onset of ‘turbulence’ at Rayleigh numbers in the region of 5 × 10 4 to 10 5 marks the transition from laminar Bénard-Rayleigh heat flow to quantised flow by thermals.