The stability of the clinoptilolite-rich tuff from Oaxaca (México) treated with acid phosphate solutions at pH values from 0 to 6 at 70 °C was investigated with base on the dealumination of the zeolitic material and the effective ion exchange capacity property. The aluminum of the natural zeolite was determined by both atomic absorption spectroscopy and near infrared spectroscopy developed in this work. The effective cation exchange capacity (EfCEC) of the clinoptilolite-rich tuff was determined based on the method described by Ming et al. According to the experimental data, the pH of the phosphate solutions influences the stability of the clinoptilolite-rich tuff. The dealumination of the zeolitic material at pH 0 follows a first order kinetic model. It was also found that the EfCEC is the lowest and the material loses crystallinity at the same pH. The presence of phosphorous in the zeolitic material is observed after the treatment with the acid phosphate solutions. Therefore, the treatments of clinoptilolite-rich tuff from Oaxaca with the acid phosphate solutions modify its characteristics depending on the acidity of the aqueous media and consequently its stability varies.