An investigation of 20th-century Customark chemical watermarks beginning with methods of identification is reported. Experimentation with various conservation treatment steps on these materials gave conflicting results. Erasers seemed not to affect the watermark. Organic chemicals quickly altered the chemical watermark visually, yet under ultraviolet examination the mark appeared unaltered. Conversely, exposure to other solutions left the watermark visually undisturbed yet indistinct under ultraviolet illumination. It is hypothesized that component parts of the chemical watermark and perhaps paper brighteners, fillers, and/or sizing agents were moved by treatment methods, altering the mark.