Adiabatic decompression may result in the heating of geologic aqueous fluids on the order of tens of degrees Celsius. Adiabatic expansion may be either reversible, in which case geologically important fluids cool on decompression, or irreversible. If the irreversible adiabatic expansion involves a significant pressure loss due to internal work generation (and kinetic and gravitational effects can be ignored) and the Joule-Thompson coefficient is large and negative, fluids heat up upon decompression. The Joule-Thompson coefficient becomes increasingly negative (i.e., the heating effect dominates) at high pressures, low temperatures, high mNaCl, and high XCO2. We predict that high concentrations of divalent and trivalent ions in solution will also increase the magnitude of the heating effect.