Chemically modified and natural zeolite consisting of clinoptilolite and mordenite minerals (Sokirnica Mine, Ukraine) was selected for testing the adsorption ability of formaldehyde. Modified zeolite was produced from natural zeolite by chemical activation with sulphuric acid (H₂SO₄) 2M at 95-98°C for six hours. Synthetic wastewater was prepared by adding formaldehyde at concentrations equal to or higher than the maximum allowable concentration (MAC) values: 1 MAC = 2 mg/l, 2 MAC = 4 mg/l, 3 MAC = 6 mg/l, 5 MAC = 10 mg/l, and 10 MAC = 20 mg/l. The effects of adsorption time (5 min, 10 min, 30 min, 60 min, 3 h, 5 h, 8 h, and 12 h) were examined for the selected formaldehyde concentrations. Adsorption efficiency, adsorption capacity, and iodine numbers were evaluated for the selected adsorbents. The positive effect of natural zeolite chemical modification was obtained in the range of 5.4 to 24.0%, with an average value of 9.2 %. Iodine number was selected as a parameter for natural zeolite chemical modification with 2M sulphuric acid evaluation. The iodine number before sorption was 154.3 mg∙g^-1 and after modification was 164.5 mg∙g^-1.