An ophthalmic pharmaceutical composition containing acetylcysteine as the active ingredient having a mucolitic action, suitable for administration in eyedrops, where N-acetyl-cysteine is neutralised with base DEAE-dextrane in order to reach pH levels preferably between 6.0 and 7.5, and having a physiologically acceptable osmolarity less than 320 mOsm/kg. The presence of a neutralising agent for N-acetyl-cysteine that does not negatively affect the composition osmolarity allows the formulation to be isotonic or even hypotonic, thus avoiding the irritative and potentially damaging effects of previous ophthalmic products containing N-acetyl-cysteine.