The Stiff and Davis stability index (S&DSI) has never had a clear thermodynamic or geochemical definition. In the present work, an analysis of its formulation allowed it to be defined as the saturation index (SI) of a calcium carbonate polymorph, investing it with clear thermodynamic significance. This analysis also allowed the K coefficient, an essential component of the S&DSI, to be defined; until now, no clear definition has been available. Finally, the experiment performed by Stiff and Davis that led to their announcement of the S&DSI was reproduced by geochemical modelling. The results obtained indicate that calcite was probably the calcium carbonate polymorph used in the construction of this index. (C) 2012 Elsevier B.V. All rights reserved.