BACKGROUND In regard to patients hypersensitive to p-tert-butylphenol formaldehyde resin (PTBP-F-R), it is for diagnostic, therapeutic, and preventive reasons necessary to know the identity of the primary sensitizing substances and their sensitizing capacities, as well as their cross-reaction patterns. In patients hypersensitive to PTBP-F-R, we have demonstrated a statistically significant overrepresentation of simultaneous reactions to p-tert-butylcatechol (PTBC). In the guinea pig, we have shown that PTBC is a strong sensitizer, giving cross reactions to p-tert-butylphenol. Furthermore, PTBC cross reacts to the PTBP-F-R monomer 2,6-methylol p-tert-butylphenol, which is a strong sensitizer in the guinea pig. OBJECTIVE The aim of this study was to investigate the background to the observed simultaneous reactions to PTBP-F-R and PTBC and to see if these could be explained by the presence of PTBC in PTBP-F-R. METHODS High pressure liquid chromatography (HPLC), nuclear magnetic resonance spectrometry (NMR), mass spectrometry (MS). RESULTS PTBC was isolated and identified; the concentrations in two resins were determined. HPLC analyses showed the presence of 0.099% and 0.020% wt/wt PTBC, respectively. CONCLUSIONS This study shows that PTBC can be present in at least in some brands of PTBP-F-R. The study also indicates that a positive patch test reaction to PTBP-F-R could be an indication of allergy to PTBC, at least in patients with high sensitivity to PTBC. When detecting contact allergies to PTBP-F-R in a patient for whom no clinically relevant exposure to this resin can be found, the possibility of PTBC as the eliciting factor should be considered.