Equilibrium constants for bromine hydrolysis, K-1 = [HOBr][H+][Br-]/Br-2(aq)], are determined as a function of ionic strength (mu) at 25.0 degrees C and as a function of temperature mu similar or equal to 0 M. At mu similar or equal to 0 M and 25.0 degrees C, K-1 = (3.5 +/- 0.1) x 10(-9) M(2) and Delta H degrees = 62 +/- 1 kJ mol(-1). At mu = 0.50 M and 25.0 degrees C, K-1 = (6.1 +/- 0.1) x 10(-9) M(2) and the rate constant (k(-1)) for the reverse reaction of HOBr + H+ + Br- equals (1.6 +/- 0.2) x 10(10) M(-2) s(-1). This reaction is general-acid-assisted with a Bronsted alpha value of 0.2. The corresponding Br-2(aq) hydrolysis rate constant, k(1), equals 97 s(-1), and the reaction is general-base-assisted (beta = 0.8).