Kinetics and products of the photoreaction of the phenyl ketone valerophenone were investigated as a function of temperature, pH, and wavelength in aqueous solution. Under these conditions (<10(-4) M), the photoreactions are pseudo-first-order with respect to valerophenone concentration. Type II quantum yields for photoreaction were close to unity throughout the 290-330 nm spectral region and ill the temperature range from 10 to 40 degrees C. The quantum yields for the photoproducts were 0.65 +/- 0.04 for cleavage to acetophenone and propene and an overall yield of 0.32 +/- 0.03 for cyclization to two cyclobutanols at 20 degrees C. A small amount of 1-phenylcyclopentanol (similar to 2% yield) also was formed. These photoreactions were quenchable by additions of the triplet quenchers sorbic alcohol or sorbic acid, and Stern-Volmer plots were Linear up to at least 80% quenching of the photoreactions. On the basis of quenching studies with steady-state irradiations, the triplet lifetime of valerophenone at 20 degrees C was estimated to be 52 ns, similar to 7 times longer than that observed in hydrocarbon solvents. Since the triplet lifetime is controlled by intramolecular hydrogen abstraction, these results indicate that the rate constant for H abstraction is significantly lowered in aqueous media. The slower H abstraction in aqueous solution is attributed to stabilization of the excited pi,pi* state by water and vibronic mixing and slight inversion of the reactive n,pi* triplet and the unreactive pi,pi* triplet states. This interpretation also is supported by changes in the UV absorption spectra of phenyl ketones in water compared to organic solvents. Red shifts, compared to the polar- organic solvent acetonitrile, were observed,in the pi-pi* transitions of valerophenone and acetophenone, reflecting stabilization of the excited pi,pi* state by water. Other results indicated that the quantum yields for valerophenone photoreaction are pH-independent from pH 9 to pH 2 but decrease significantly below pH 2. The decrease at low pH is attributed to quenching of triplet reactivity via protonation of the excited triplet state. The use of valerophenone as a convenient actinometer for studies in water is discussed; its half-lives during midday exposure to summer sunlight in temperate latitudes are <30 min.