The thickness stability of the film between air bubble/oil drop and oil/oil drops in an induced air flotation process in the presence of flocculent was analyzed. A cationic, high molecular weight, polymeric flocculent was used in the demulsification of a crude oil-in-water emulsion. A novel capillary force balance technique in conjunction with the him interference microscopy was used to study the film thickness stability of an emulsion Nm formed between oil drops and pseudoemulsion film formed between air bubbles and oil drops. It has been found that film thickness stability is related to the formation of dark spots (thinner area) inside the him. The formation of dark spots inside the thinning film has been explained by the local osmotic depletion-destabilization effect. It has been found that in the presence of the flocculent the emulsion film is less stable than the pseudoemulsion film, and the efficiency of the oil separation process by air flotation is therefore governed by the stability of the water emulsion film. The flocculent improves oil spreading on the water-air surface, thereby enhancing separation efficiency.