The study focuses on the presence of biocolonization on the Fisher Fine Arts Library, where two visually distinct types of biocolonization are found, green and black. The former grow in the damper, lower courses, while the latter occurs in the middle courses. The top courses of the apse are protected from rain so no apparent colonization is seen there. Biodeterioration is any undesirable change in the properties of a material caused by the vital action of organisms. The most used methods to control and prevent biodeterioration are to eliminate biocolonization through the application of biocides. The presence of biocolonization is not only an aesthetic issue, but one of deterioration, because microorganisms can alter the material both physically and chemically making it more susceptible to other deterioration mechanisms. The study assesses the efficiency of the removal of the two types of biocolonization by two different biocides, D/2 Biological Solutions and Enviro Klean BioWash. The effectiveness of the two biocides was evaluated using a thermal imager by wetting the stone to determine how the presence of biocolonization affects the absorption/ evaporation of water. The temperature information obtained from the thermal imager was interpreted to determine which biocide was the most effective. Visual inspection and RILEM tube testing were also used to aid in the evaluation of the biocide. The study showed that the green biocolonization was effectively removed from the stone by both biocides, however, the black biocolonization was not and this requires a more in depth study to understand it since the black coloration could also be the result of the formation of an iron oxide patina.