First, we attempted to synthesize a DD3R zeolite membrane by using precursor solutions containing small amounts of a structure-directing agent and potassium fluoride (KF), with the aim of obtaining a membrane with high permeance toward CO2. The most dilute precursor solution (1- adamantylamine:KF:SiO2:H2O = 0.026:0.026:1:52) produced a thin and dense zeolite membrane that showed a high CO2 permeance of 12 x 10(-8) mol m(-2)S(-1)Pa(-1) and a high PCO2/PCH4 of 270 at a pressure drop of 1.0 MPa. We then studied the synthesis of DD3R membranes with various alkali metal halides (KCl, KBr, NaF, NaCI, or NaBr) as the mineralizing agent (MA). Regardless of the alkali metal halide selected, the addition of a small amount of MA permitted the preparation of DD3R membranes with a high separation selectivity of 190-380 at a pressure drop of 1.0 MPa. Addition of excessive amounts of MA caused gelling of the solution and a decrease in the separation selectivity. The presence of a few alkali metal cations promotes the growth of zeolite crystals on the support and permits the synthesis of dense DD3R zeolite membranes.