Nanosecond laser flash photolysis of o-hydroxyacetophenone (1a) and 2,4-dihydroxyacetophenone (1b) in ethanol and acetonitrile results in absorption due to triplet biradicals 2a (lambda(max) 430 nm, tau approximate to 3 mu s) and 2b (lambda(max) 400 nm, tau approximate to 1 mu s), respectively. Triplet biradical 2a intersystem crosses to form Z-3a (lambda(max) 400 nm, tau approximate to 10 mu s), whereas 2b forms both Z-3b and E-3b (lambda(max) 350 nm, tau approximate to 5 and 72 mu s). Quenching studies demonstrate that 3a,b are formed on both the singlet and triplet excited surface of 1a and 1b. In ethanol at 77 K, o-hydroxyacetophenone derivatives 1a and 1b show phosphorescence, as is typical for triplet ketones with (n,pi*) configuration. The mechanism for the photoreactivity of 1a,b is supported by density functional calculations.