The thermal decomposition of acetaldehyde, CH3CHO + M -> CH3 + HCO + M (eq 1), and the reaction CH3CHO + H -> products (eq 6) have been studied behind reflected shock waves with argon as the bath gas and using H-atom resonance absorption spectrometry as the detection technique. To suppress consecutive bimolecular reactions, the initial concentrations were kept low (similar to 10(13) cm(-3)). Reaction 1 was investigated at temperatures ranging from 1250 to 1650 K at pressures between I and 5 bar. The rate coefficients were determined from the initial slope of the hydrogen profile via k(1) = [CH3CHO](0)(-1) x d[H]/dt, and the temperature dependences observed can be expressed by the following Arrhenius equations: k(1)(T, 1.4 bar) = 2.9 x 10(14) exp(-38 120 K/T) s(-1), k(1)(T, 2.9 bar) = 2.8 x 10(14) exp(-37 170 K/T) s(-1), and k(1)(T, 4.5 bar) = 1.1 x 10(14) exp(-35 150 K/T) s(-1). Reaction 6 was studied with C2H5I as the H-atom precursor under pseudo-first-order conditions with respect to CH3CHO in the temperature range 1040-1240 K at a pressure of 1.4 bar. For the temperature dependence of the rate coefficient the following Arrhenius equation was obtained: k(6)(T) = 2.6 x 10(-10) exp(-3470 K/T) cm(3) s(-1). Combining our results with low-temperature data published by other authors, we recommend the following expression for the temperature range 300-2000 K: k(6)(T) = 6.6 x 10(-18) (T/K)(2.15) exp(-800 K/T) cm(3) s(-1). The uncertainties of the rate coefficients k(1) and k(6) were estimated to be +/- 30%.