A treatment for fungus growth on paper-based materials that would not only kill existing growth but would also inhibit future spore development would be of great utility; any such treatment should also be innocuous to the objects. Ortho-phenylphenol (opp) and thymol were tested. Thymol is more commonly employed but concern over the possible health hazard it poses is growing. The fungi chosen as test organisms represent a range of taxonomic groups, spore morphology, spore production and growth rates. All flourish on paper and produce noticeable pigmentation. Neither fungicide produced 100% fungicidal nor 100% fungistatic action, which would suggest that neither is an effective method for treating mouldy books and papers. This is in conflict with other work testing thymol. The authors conclude that lowering ambient relative humidity in libraries and archives is still the only sure way to eliminate mould growth. References.