MINEQL: A Compact Program for Computation of Chemical Equilibria in Aquatic Systems. This report describes the computer program for the computation of chemical equilibria in aqueous systems, MINEQL. The purpose of the report is to present the program in sufficient detail that the reader not only can use the program exactly as it is written, but also can modify it to suit whatever his particular needs may be. The evolution of the program, examples of its use, and general approach to the chemical equilibrium problem are described in Part I of the report. Part II presents a ”MINEQL-1 User’s Manual”. A detailed mathematical description of the solution to the chemical equilibrium problem used in MINEQL appears in Appendix 1. It was felt that the solution could best be presented by an abstract description of the mathematical problem (without any reference to its chemical analog), followed by a very simple example of a chemical problem (which is given in Appendix 2). This avoids the intermingling of chemical and mathematical concepts which is sometimes confusing. Appendix 3 gives a description of the program itself. The three appendices are related in that they present the same concepts - the actual theory and its application in MINEQL - from three different viewpoints: mathematical, chemical, and computational. They should be read in conjunction with one another. First the concepts involved in the basic chemical equilibrium problem, (i.e., where there are no solid phases present) should be mastered, and then the extension to the consideration of solid phases, which is somewhat more complicated, should be undertaken. This report contains a complete description of all of the concepts employed in the program MINEQL. However, a working knowledge of the program is obtainable without attention to all of the details. It is hoped that this roadmap through MINEQL will enable the reader to maximize his use of the program without becoming bogged down in the details of the report.