ABSTRACT:- Gypseous soils contains either Sodium or Calcium salts, which may be considered the most problematic material for foundation engineer. It is associated with settlement problem especially for heavy and hydraulic structures with the presence of water. The risk begins from the first period of wetting or soaking these soils from any source: (rainfall, rising of water table or from any reason). The water will fluctuate or infiltrate through the soil particles and dissolve gypsum particles that fill the voids in the soil. The soil particles will roll and slide and arrange at new positions, because of the disintegration of the soil skeleton and the loosening of soil with the continuous dissolution of gypsum, which translate the soil from solid to semi solid media, with time.The dissolution of gypsum depends on many factors (gypsum content, temperature, atmospheric pressure, and other factors). Another important factor which is the acidity of the dissolves liquid must be considered. This study shades the lights on the effect of Acetic acid (CH3COOH) on the collapsibility of gypseous soil. A laboratory model includes 350mm diameter and 400mm height thick plastic container and 18.4kN/m3 density gypsiferous soil prepared locally and compacted in three layers, with70%gypsum content. The stress fixed at 47kPa was applied over 50mm diameter circular footing. The relation between the soaking time and the deformation ratio (settlement/width of footing S/B%) was investigated, with 5 cycles of soaking by the Acetic acid.The results of laboratory model tests show a pronounce effect of the acidity on the collapsibility of gypsiferous soil. The results of deformation ratio S/B% was (1.22, 8, 12, 15.7, 50%) at the end of (1,2,3,4,5) prewetted cycles by CH3COOH acid, respectively. So the effect of Acetic Acid percent was considerably accelerating the collapsibility of gypsiferous soil with cycling technique.Keywords:- gypsiferous soil, compressibility.