Partition coefficients relate the amount of a trace element that enters a growing crystal to the amount of that element in the coexisting fluid. These coefficients are constants and are characteristic of each mineral species. Although poorly known at the present time, partition coefficients can be readily determined in the laboratory for many geologically important minerals. When the coefficient is known, the composition of the fluids in which a crystal grew can be calculated by measuring the level of concentration of that element in a mineral. In this way new information can be obtained for important paleoecological and other geological problems. Examples are given to illustrate how this concept can be applied by using the trace elements in the evaporation products of or in contact with sea water to determine paleosalinities and related problems.