Self-reaction of hydroxyl radicals, OH + OH -> H2O + O (1a) and OH + OH -> H2O2 (1b), was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 298-834 K temperature and 1-100 bar pressure ranges (bath gas He). A heatable high-pressure flow reactor was employed. Hydroxyl radicals were prepared using reaction of electronically excited oxygen atoms, O(D-1), produced in photolysis of N2O at 193 nm, with H2O. The temporal behavior of OH radicals was monitored via transient absorption of light from a dc discharge in H2O/Ar low-pressure resonance lamp at ca. 308 nm. The absolute intensity of the photolysis light was determined by accurate in situ actinometry based on the ozone formation in the presence of molecular oxygen. The results of this study combined with the literature data indicate that the rate constant of reaction la, associated with the pressure independent component, decreases with temperature within the temperature range 298-414 K and increases above 555 K. The pressure dependent rate constant for (1b) was parametrized using the Troe expression as k(1b,inf) = (2.4 +/- 0.6) x 10(-11)(T/300)(-0.5) cm(3) molecule(-1) s(-1,) k(1b,0) = [He] (9.0 +/- 2.2) x 10(-31)(T/300)(-3.5 +/- 0.5) cm(3) molecule(-1) s(-1), F-c = 0.37.