The procedure described here is applicable for determining edetic acid (ethylenediaminetetraacetic acid) at the 0.05–0.2% level in ophthalmic solutions. This method employs a spectrophotometric titration, using a magnesium-ion solution as titrant and arsenazo I as the indicator. The chemistry involved is an adaptation of a procedure to determine water hardness using edetic acid as titrant. The apparatus employed is an automated system utilizing a probe colorimeter to monitor the titration. Good precision was demonstrated with relative standard deviations of less than 1%. The method was also shown to be applicable to commercially available ophthalmic solutions, either decongestants or hard contact lens cleaning solutions.