Browning is one of the most common postharvest changes in button mushrooms, which often results in economic losses. Phenolic compounds, which are associated with browning, were extracted from the non-bruised and bruised skin tissue of various button mushrooms with a sulphite containing solution and analysed with UHPLC-PDA-MS. In total, 34 phenolic compounds were detected. Only small differences in the total phenolic content between bruising-tolerant and sensitive strains were observed. The contents of γ-L-glutaminyl-4-hydroxybenzene (GHB) and γ-L-glutaminyl-3,4-dihydroxybenzene (GDHB) correlated with bruising-sensitivity, for example a R(2) of 0.85 and 0.98 was found for non-bruised brown strains, respectively. In non-bruised skin tissue of the strains with brown caps, the GHB and GDHB contents in sensitive strains were on average 20 times and 15 times higher, respectively, than in tolerant strains. GHB and GDHB likely participate in the formation of brown coloured GHB-melanin, which seemed the predominant pathway in bruising-related discoloration of button mushrooms.