8-Hydroxydaidzein is a suicide substrate of mushroom tyrosinase with potent irreversible inhibitory activity. Despite its high potential in the cosmetics industry, it was found that 8-hydroxydaidzein was unstable in a formulated cream. In this technical note, the stability of 8-hydroxydaidzein in various solutions is investigated. The compound was dissolved in a series of solvents, and the residual 8-hydroxydaidzeins in the prepared solutions were sequentially determined during storage by HPLC. As a result, the loss in time of 8-hydroxydaidzein in both pH 6.8 phosphate buffer and DMSO showed typical first-order kinetics, and the loss rate constant of the compound in pH 6.8 phosphate buffer (4.48 x 10(-3) hour(-1)) was 18-fold higher than that in DMSO (2.5 x 10(-4) hour(-1)). The stabilities of the compound in different buffers with pH values ranging from pH 5 to pH 9 were determined in advance. The results showed that the compound was completely degraded in one day in the pH 8 and pH 9 buffers. In contrast, 8-hydroxydaidzein remained stable above 85% after 20 days' storage in the pH 5 and pH 6 buffers. In addition to the residual 8-hydroxydaidzein analysis, the residual bioactivities, including tyrosinase inhibitory activity and DPPH-radical scavenging activity of the 8-hydroxydaidzein solutions after 20 days' storage in different pH values, were also determined, and the results correlated well with those of the stability experiments. All the results demonstrated that 8-hydroxydaidzein is unstable in alkaline solutions. According to the data in the present report, it is recommended that 8-hydroxydaidzein be formulated in an acid solution for its applications.