Hydrophobic monolayer films of dimethyldioctadecyl-ammonium bromide (DMDOA) deposited on mica by Langmuir-Blodgett (LB) method have been studied using Wilhelmy plate type wetting measurements, and atomic force microscopy (AFM). The structural change of DMDOA monolayer was observed when exposed to salt solution. The effect of exposure time to aqueous KBr salt solutions (immersion time) was especially investigated on surface morphology and wettability of DMDOA films. AFM images have shown that the surface coverage of DMDOA monolayer is decreased with increase in KBr concentration, or in immersion time to KBr solution. The wetting measurements have demonstrated that the hydrophilicity of DMDOA sample is increased with decrease in surface coverage. Drastic hydrophilisation was observed between 60% and 50% covered DMDOA samples.