A @P04794@ mechanism in which one of the @M03986@, C, is already present in an @E02090@ with A during formation of B from A, but does not assist the formation of B, e.g. The formation of B from A may itself be a @M03989@ reaction with some other @R05190@. Since C does not assist the formation of A, it is described as being present as a spectator, and hence such a mechanism is sometimes referred to as a spectator mechanism.