The reactions of H and D atoms with H2CO (H + H2CO --> H-2 + HCO (1.1), D + H2CO --> HD + HCO (2.1), and D + H2CO --> H + HDCO (2.2)) have been studied in the temperature range 296 K less than or equal to T less than or equal to 780 K in an isothermal discharge now reactor with EPR detection of D and H atoms and LIF detection of HCO. Simultaneous measurements of the absolute concentration-time profiles of the three species established the occurrence of the D/H isotope exchange reaction (2.2) in addition to the H atom abstraction channel (2.1). The rate constants for the three reactions could be represented by the Arrhenius expressions k(1.1)(T) = (8.7 +/- 1.9) x 10(12) exp[-(14.5 +/- 0.7) kJ mol(-1)/RT] cm(3) mol(-1) s(-1), k(2,1)(T) = (1.2 +/- 0.5) X 10(13) exp[-(15.8 +/- 0.8) kJ mol(-1)/RT] cm(3) mol(-1) s(-1), and k(2,2)(T) = (5.9 +/- 1.5) x 10(12) exp[-(14.7 +/- 1.0) kJ mol(-1)/RT] cm(3) mol(-1) s(-1). A mechanistic analysis of reaction 2.3 using the unimolecular rate theory gave these estimates for the classical potential energy barrier heights in the addition of D and H atoms to H2CO: DeltaE(0)(D + H2CO) = 1360 +/- 100 cm(-1) and DeltaE(0)(H + H2CO) = 1540 +/- 150 cm(-1).