Abstract: 2.000 g of air-dried soil sample having passed through a sieve with 2 mm bore were taken and pre-treated by an automatic rinsing device with 80 mL of 1.00 mol·L^-1 ammonium acetate solution, to exchange all the cations in the soil sample by NH₄⁺, after which the soil sample was rinsed repeatedly with ethanol. After rinsing, the soil sample was transferred to a distiller to distill ammonia from the sample, and the distillate was led to a colorimetric tube in which 20 mL of 20 g·L^-1 boric acid solution was added as absorbent. When the solution attained to ca. 90 mL during distillation, the colorimetric bute was removed from the distiller, and volume of the solution was made up to 100.0 mL with water. Content of NH₄-N in the absorption solution was then determined by gas molecular absorption spectrometry (GMAS). The absorption solution can be kept as long as 24 h before the spectrometric determination. No significant deviation of results of NH₄-N was found for determination in this absorption solution with preservation time within 24 h. Standard NH₄⁺-N solution was used for preparation of standard curve (in the linearity range of 0.10 to 2.00 mg·L^-1) by the GMAS method. Values of cation exchange capacity (CEC) were calculated by the formula given based on data of NH₄⁺-N found. The proposed method was tested by analyzing 4 CRMs of soil, giving values of CEC in consistency with the certified values. RSDs (n=6) found for these 4 samples were all less than 2.0%.