The effect of a commercial flocculant (Alpacon (R) WS009) and two coagulant salts (CaCl2 and AlCl3) on the stability of metalworking oil-in-water (O/W) emulsions was examined. Two O/W emulsions were tested: a fresh emulsion, prepared in the laboratory from a commercial concentrate, and a waste metalworking emulsion, provided by a local waste management company, with initial oil concentrations of 32900 and 16900 mg/L, respectively. The emulsion stability was studied at different demulsifier concentrations, temperatures and pH through centrifugation tests, zeta potential and multiple light scattering measurements. Emulsion breakdown is explained by electrostatic repulsion of oil droplets and steric interactions. The former was observed for the laboratory emulsion, while the latter was observed for the waste emulsion. Aluminum chloride was the only effective agent for demulsifying both emulsions.