Hypocrellin A (HA) is an efficient phototherapeutic agent. Illumination of HA in dimethylsulphoxide (DMSO) or DMSO-buffer (1:1 by volume, pH > 6.7) generated a strong electron paramagnetic resonance (EPR) signal. This EPR signal was intensified in the presence of reductants. The EPR spectrum obtained was assigned to the semiquinone radical anion of HA (HA'-) based on a series of experimental results. Decay of HA'-, attributed to a radical-radical reaction, follows second-order kinetics. In acidic DMSO-buffer (1:1 by volume) solution, no EPR signal of HA'-was detected in the absence or presence of reductant. This was explained by a fast disproportionation of radicals, facilitated by protonation of radicals. The spectrophotometric measurements indicated that on illumination HA was directly reduced to its two-electron reduction product, i.e. hydroquinone, in acidic solution. The absorption maximum of the hydroquinone of HA which is at 496 nm at pH 5.8 shifts bathochromically with increase in pH of the medium. In DMSO or DMSO-buffer (1:1 by volume, pH>6.7) solutions the semiquinone radical anion of HA was also observed spectrophotometrically. The absorption maximum of HA'-is at around 628 nm. Strong intramolecular hydrogen bonding was considered to exist in the chromophore of HA'-.