Two specimens having identical tristimulus values, for a given illuminant and reference observer, are metameric if their spectral radiance factors differ from each other within the visible spectral range. This metameric colour match is broken down either by a change of the reference illuminant or the observer. The degree of colour mismatch caused by substituting a test illuminant of different spectral composition for the reference illuminant is classified as the Special Metamerism Index: Change in Illuminant (illuminant metamerism). The method evaluating illuminant metamerism has already been formulated (see CIE 015.2-1986): the purpose of this report is to formulate a method of evaluating the degree of colour mismatch for metameric colour pairs (object colours or illuminant colours) caused by substituting a test observer with normal colour vision for the reference observer (CIE Standard Colorimetric Observer). This is classified as the Special Metamerism Index: Change in Observer (observer metamerism). Colour mismatch caused by observer metamerism is inevitable for real observers with normal colour vision. The evaluation of observer metamerism thus gives a sound basis for establishing colour difference tolerances for colour matching using various colour pairs. It also gives an indication