Examples of compensation that fall into these three cat-egories are defined as secondary, in that they are betterexplained either in terms of well-known thermodynamiclaws (1, 2) or by which systems were selected and whichmeasurements were made (3). In contrast, extra-thermody-namic compensation is defined here as a linear relationshipbetween ⌬H and ⌬S, which does not fall into the thirdcategory above. The key quantity provided by such a rela-tionship is the slope, which defines a compensation tem-perature Tc ⳱ d⌬H/d⌬S. The implication (or hope) isthat for extra-thermodynamic compensation, Tc should re-veal some mechanistic or extra-thermodynamic informationabout the system that cannot be a priori deduced from thelaws of statistical thermodynamics. Examples might be in-formation about the shape of the potential energy surface,the distribution of energy levels available to the system, orthe interaction between different components of the system.