The rate constant of acetaldehyde thermal dissociation, CH3CHO = CH3 + HCO; was measured behind reflected shock waves at temperatures of 1273-1618 K and pressures near 1.6 and 0.34 atm. The current measurement utilized sensitive CO diagnostics to track the dissociation of CH3CHO via oxygen atom balance and inferred the title rate constant (k(1)) from CO time histories 7 obtained in pyrolysis experiments of 1000 and 50 ppm of CH3CHO/Ar mixtures. By using dilute test mixtures, the current study successfully suppressed the interferences from secondary reactions and directly determined the title rate constant as k(1)(1.6 atm) = 1.1 X 10(14) exp(-36 700 K/T) s(-1) over 1273-1618 K and k(1)(0.34 atm) = 5.5 X 10(12) exp(-32 900 K/T) s(-1) over 1377-1571 K, with 2 sigma uncertainties of approximately +/- 30% for both expressions. Example simulations of existing reaction mechanisms updated with the current values of k(1) demonstrated substantial improvements with regards to the acetaldehyde pyrolysis chemistry.