4-Hydroxy-6-methylene-2,4-cyclohexadien-1-one (1) and 4-methoxy-6-methylene-2,4-cyclohexadien-1-one (2) were generated by efficient (Φ = 0.3) photodehydration of 2-(hydroxymethyl)benzene-1,4-diol (3a) and 2-(hydroxymethyl)-4-methoxyphenol (4a), respectively. o-Quinone methides 1 and 2 can be quantitatively trapped as Diels-Alder adducts with ethyl vinyl ether or intercepted by good nucleophiles, such as azide ion (k(N3)(1) = 3.15 × 10(4) M(-1) s(-1) and k(N3)(2) = 3.30 × 10(4) M(-1) s(-1)). In aqueous solution, o-quinone methide 2 rapidly adds water to regenerate starting material (τ(H(2)O)(2) = 7.8 ms at 25 °C). This reaction is catalyzed by specific acid (k(H(+))(2) = 8.37 × 10(3) s(-1) M(-1)) and specific base (k(OH(-))(2) = 1.08 × 10(4) s(-1) M(-1)) but shows no significant general acid/base catalysis. In sharp contrast, o-quinone methide 1 decays (τ(H(2)O)(1) = 3.3 ms at 25 °C) via two competing pathways: nucleophilic hydration to form starting material 3a and tautomerization to produce methyl-p-benzoquinone. The disappearance of 1 shows not only specific acid (k(H(+))(1) = 3.30 × 10(4) s(-1) M(-1)) and specific base catalysis (k(OH(-))(1) = 3.51 × 10(4) s(-1) M(-1)) but pronounced catalysis by general acids and bases as well. The o-quinone methides 1 and 2 were also generated by the photolysis of 2-(ethoxymethyl)benzene-1,4-diol (3b) and 2-(ethoxymethyl)-4-methoxyphenol (4b), as well as from (2,5-dihydroxy-1-phenyl)methyl- (3c) and (2-hydroxy-5-methoxy-1-phenyl)methyltrimethylammonium iodides (4c). Short-lived (τ(25°)(C) ≈ 20 μs) precursors of o-quinone methides 1 and 2 were detected in the laser flash photolysis of 3a,b and 4a,b. On the basis of their reactivity, benzoxete structures have been assigned to these intermediates.