While most common chromogens are relatively resistant to organic solvents, some chromogens might be dissolved in xylene. Using xylene substitutes and ensuring that the ethanol solutions are free of xylene contaminants (a common issue if the same solutions are used for re- and dehydration) will prevent these chromogens from being washed away. Some rare chromogens are incompatible with organic solvents and should be mounted in aqueous solutions.