Metofluthrin [I, 2,3,5,6-tetrafluoro-4-(methoxymethyl)benzyl (EZ)-(1R,3R)-2,2-dimethyl-3-(prop-1-enyl)cyclopropanecarboxylate] was resistant to hydrolysis at pH 4 and 7 but moderately hydrolyzed with a half-life of 36.8 days at pH 9 and 25°C to form the corresponding acid and alcohol via ester cleavage. The activation energy was estimated to be 105 kJ mol⁻¹ based on the hydrolysis rates at 25–50^oC. By continuous exposure to light at >290 nm from a Xenon arc lamp at hydrolytically stable pH 4 and 25°C, I was rapidly photodegraded with half-lives of 1.1–3.4 days, mainly via ester cleavage and successive oxidation followed by mineralization to carbon dioxide. Spectroscopic analyses together with co-chromatography with authentic standards showed that the major degradates having an ester linkage were the aldehyde and carboxylic acid derivatives formed via oxidative cleavage of the prop-1-enyl group together with the diol formed possibly through an oxidative intermediate, such as an epoxide.