It is difficult to compare the magnitude of osmotic adjustment (OA) between species and experiments. Currently, comparisons are made by determining the magnitude of OA (MPa) at predetermined values of leaf water content, e.g. a relative water content (RWC) of 70%. The relationship between the natural logarithms of leaf water content (RWC, dependent variable) and leaf osmotic potential (pi, independent variable) has a slope of 1.0 for the concentration effect. For tissues that adjust osmotically, the slope will be less than 1.0 and the osmotic index will be 1.0 - slope. The index applies to all the patterns of n(RWC) vs ln(pi) observed in experiments and is independent of the amount of drying achieved. An index of 1.0 is full capacity for OA, and a value of 0.0 is no OA. Three examples of the application of the index are given for data published on sorghum, wheat and grape vines.