Absolute rate constants are measured for the reactions: OH + CH2O, over the temperature range 296–576 K and for OH + 1,3,5‐trioxane over the range 292–597 K. The technique employed is laser photolysis of H2O2 or HNO3 to produce OH, and laser‐induced fluorescence to directly monitor the relative OH concentration. The results fit the following Arrhenius equations: k (CH2O) = (1.66 ± 0.20) × 10−11 exp[−(170 ± 80)/RT] cm3 s−1 and k(1,3,5‐trioxane) = (1.36 ± 0.20) × 10−11 exp[−(460 ± 100)/RT] cm3 s−1. The transition‐state theory is employed to model the OH + CH2O reaction and extrapolate into the combustion regime. The calculated result covering 300 to 2500 K can be represented by the equation: k(CH2O) = 1.2 × 10−18 T2.46 exp(970/RT) cm3 s−1. An estimate of 91 ± 2 kcal/mol is obtained for the first CH bond in 1,3,5‐trioxane by using a correlation of CH bond strength with measured activation energies.