An in vitro method for determining the bioavailability of zinc (II) ions from an oral care composition comprises: providing a sample comprising, the oral care composition; contacting the sample with a biorelevant substrate, separating the biorelevant substrate from the sample, contacting the biorelevant substrate with an indicator under conditions selected such that the indicator is capable of reacting with zinc (II) ions, and determining a light absorbance of the indicator. The indicator is zincon or a salt thereof. The method is useful for estimating the ability of an oral care composition to deliver therapeutically-active zinc (II) ions to the oral cavity. Also provided is the use of zincon and a biorelevant substrate in vitro to determine the bioavailability of zinc (II) ions from an oral care composition comprising, a zinc source.