The kinetics of the reaction of mercuric acetate with alkenes in methanol can be measured directly by observing the ultraviolet absorbance of the solution at 265 nm. For more reactive alkenes stopped flow techniques are required, but for less reactive substrates conventional spectrophotometers are adequate. The measured rates are in satisfactory agreement with values obtained by other methods. The rates of several cyclopropylalkenes were measured in this way, and relative reactivities of some of the same substrates were obtained by the competition method. Rates obtained by both methods are in qualitative agreement, and show very high reactivity for the cyclopropylalkenes. These rates are analyzed by comparison with model reactions involving open ions (acid-catalyzed hydration) and bridged ions (sulfenyl halide addition). The comparisons suggest that the oxymercuration of alkenes not substituted with strongly resonance electron-donating substituents, for example ethylene, proceeds through bridged rate-determining transition states that put very little positive charge on carbon, but that when strongly resonance electron-donating substituents such as cyclopropyls are present, the rate-determining transition state resembles an open carbonium ion.