A dilution technique is described which permits a determination of the ionization constant of amines by a conductance method, in a closed system. Small amounts (5–6 g) of the amine salt are required, making the method useful in studying isotope effects. Values of Ki for ethylamine over the temperature range 1.5–45.3 °C led to calculated values at 25 °C of ΔG = 4528.3 ± 0.8 cal/mole, ΔH = −266 ± 12 cal/mole, ΔS = −16.08 ± 0.04 cal/mole deg, ΔCp = −62.2 ± 1.8 cal/mole deg.